I've been meaning to read about 'Keyline', when Darren Doherty mentioned you can read it online here, I'm glad I did. If you don't have time to read the book yourself, but still want to hear the explanations in his words, here are some quotes:
Chapter 2
"(1) Rainfall on or near a valley rapidly concentrates in the valley and flows off the area not only preventing the ridges from absorbing their fair share of the rainfall, but in poor soil, taking with it some of the soil from both valley and ridge.
(2) Keyline cultivation is in effect many hundreds or thousands of very small absorbent drains, preventing rainfall from concentrating in the valley--thus resisting and offsetting the natural rapid concentration of this water into the valleys."

"Keyline cultivation is not strictly contour cultivation.
  It is rather an "off the contour" type of cultivation, which in no small measure depends for its effectiveness on this planned drift away from the valleys."

"Keyline diffuses rainfall evenly over the whole of the land to absorb it in the greatest water conservation storage area--the land itself."

"Keyline planning can be applied on an area of virgin grassland or forest to develop it into a farming or grazing property. In timbered country it plans the clearing to retain timber in the best places; it positions the house or homestead, all other farm buildings, entrance and farm roads, large and small paddocks, dam sites and irrigation areas. It guides the whole course and sequence of development as well as the details of all cultivation for soil fertility improvement and high yields."
Chapter 3
" The first requirement, already stated, is the retention of all rainfall in the land for the production of fertility, and not methods to "safely" allow water to leave the property."

"There is little evidence anywhere in nature to support the "take and put" theory of farming where farmers are taught to "put back" into the soil each year what they "take out" in crops. So much of what is taken out is composed of materials that are available in unlimited supply from the sun, air and moisture--moisture alone requiring conservation--that if farmers cease to "mine" the top inches of the soil and farm the land, little if anything else need be put back."

"The processes which developed natural fertile soil are capable of control and tremendous acceleration."

" Man and his machines can stimulate decay and growth tremendously.
  When vegetation is stirred into the aerated part of the soil, decay continues for a longer period. Moisture remains longer to supply the needs of decay."

"Some soil scientists estimate that there are 70 tons of living organisms and other life in an acre of fertile soil. These organism generally work towards man's health and well-being.
  The importance of fourteen five-ton truck loads of microbes in an acre is overshadowed completely by a sheep or two to the acre. The sheep or cattle obviously need constant care, but surely this other "livestock" warrants some conscious thought when it is so vital. All the elements of growth are made available to us by the various processes of the life cycles of this "life in the soil". Soil management can reduce this dynamic force to a low ebb, or tremendously stimulate its activities."

"Extremely fine "seed-beds" are still produced on some farms, almost as if the crop in its growth was expected to devour every fine soil particle.
  Too fine a cultivation destroys the soil's structure, smothers and reduces soil life, thus degenerating the art of soil management into a bandit-life process of fertility extraction."

"However, it takes an appreciable time to destroy fertile soil. By wrong cultivation methods, farmers have been able to produce vast quantities of grain for generations during the process of fertility extraction.
  Now this soil has lost its former structure and its capacity to absorb fertility. Its destruction is much more rapid. Usually only three inches or a little more of the earth was used, while the basic materials for the improvement of fertility existed both above and below in limitless quantities."

"is a means of inducing or producing fertile soil over large areas of land quickly and profitably. It is only necessary to maintain the soil in a condition to absorb all the vast fertility potential of the sun, air and rainfall. If the rainfall is absorbed into the earth, the store of minerals, chemicals and gases it carries always is filtered out of it and retained in the soil."

Chapter 4

"New Keyline Absorption-fertility cultivation is practically erosion Proof; within a year or two of the resulting improvement to the soil, it is certainly so."

"The too deep cultivation of sticky clay subsoil is waste of time and money. It will seal immediately rain falls. There is generally little purpose and no profit in cultivating to depths that cannot be held by definite soil improvement."

Chapter 5

"Single working Keyline cultivation with a depth of penetration just through the top soil into the compacted zone is a logical first means to supply both the air and moisture required. Excellent results will follow this work completed in the autumn. Another suitable time is probably a few weeks before each locality's best rainfall season."

" It is often highly profitable to conduct a two- or three-year plan for the improvement of a very poor soil paddock.
  First Year.--One Keyline cultivation working approximately 4-1/2 inches deep with spike spacing 12 inches apart is given in the autumn.
  Second Year.--Another Keyline cultivation working five to seven inches deep with spike spacing 24 inches apart.
  Third Year.--A further Keyline cultivation working seven to ten inches deep with spike spacing 36 inches apart."

" The drastic deep ripping or subsoiling of pastures on the poorer thin soils, while probably increasing first year yield, will all too often be disappointing in yield for following years. Deep ripping with rigid implements is very costly and throws up clods which are too big. Heavy soil will not remain open to this depth but will reseal with the first good rain. There is no profit in taking depth that cannot be held. The topsoil fertility will fail to produce a rapid soil change in the subsoil if it is given too much depth of subsoil to "convert". Again consider the topsoil as a yeast and do not subject it to too great a dilution--as may take place in the case of overall deep ripping or subsoiling."

"The present methods of subsoiling crop land, where deep sub-soilers rip the soil to 24 inches deep, and surface cultivating implements follow, is wrong. The fine surface cultivation of deep subsoiling largely offsets the benefits of the moisture and air absorption capacity of the subsoiling. All the benefits of subsoiling, without its usual disadvantages and high cost, are obtained in the final deeper run of Keyline cultivation. Extra depth can be obtained by increasing the cultivating row spaces.
  The object of Keyline cultivating below the soil into the subsoil is always the improvement of soil fertility and the conversion of this subsoil into more fertile soil. It can be done most profitably and economically only as a progressive process."

"One low cost fast run with spikes or chisels on the Keyline principle makes available all the ingredients for a new fertility. Within a few weeks after rain on this cultivation, the return of life to the soil and pasture can be seen in the rapidly changing structure of the soil."

"The area shown in the lower half of the picture was originally pasture sown on shallow disking. After this one Keyline cultivation, pasture growth improved fourfold."

Chapter 6

"A row of stakes first marks the Keyline. Leave a narrow strip unploughed on each side of the Keyline stakes. On this, brush or trees will grow along the line during the time the area is closed to stock for cropping. This line need only be a few feet wide and it will serve as a permanent marker for the Keyline.
  Without the tree growth on the unploughed Keyline strip, a marker can be satisfactorily preserved by carefully following the lines of the previous cultivation.
  Another means of permanently marking the Keyline is to use it as a farm roadway."

Chapter 7

"  IT has already been stated that the greatest available water storage capacity exists in the soil itself. The association of Keyline cultivation and this water storage capacity has already been explained."

"The application of Keyline methods requires very little levelling work, but those levels that it does require are of great importance."

"On undulating country, dams can usually be located which will enable the farmer to enlist the forces of gravity to provide him with water under pressure. This will give him a better farm, easier work and higher yields. Other things being equal, the value to a farmer of conserved water is in direct proportion to the height of the storage. The dams of potentially greatest value are those in his high country."

"With the Keyline positioning the highest suitable dam sites, it becomes important to locate potential water-shedding areas above the Keyline."

" It is usually convenient and good practice from most other viewpoints, to locate the homestead and all farm buildings and the yards and their attendant roads in the higher country. From the point of view of full Keyline development, it becomes a part of planning to do so, in order to secure abundant run-off water to fill the Keyline dams from these sources."

"Where areas of land exist that are 50 feet or more vertically lower than the Keyline, the water from the Keyline dam will supply effective pressure for irrigation without pumping. This "line of effective water pressure" suitably forms the top boundary for the irrigation paddocks. A 4-inch pipe through the wall, controlled by a 4-inch gate valve, in these circumstances will control gravity pressure which, often from a single dam, will effectively operate a comprehensive spray irrigation and stock-watering system."

" The back of the wall of the dam, that is the side away from the water, should not be specially compacted. If water seeps through the compacted front of the wall into the centre, it must be allowed to get out through the back of the wall, otherwise it may build up hydrostatic pressure inside the wall. This could destroy the wall by forcing or breaking the material from the back of it.
  Clean water seeping through a dam wall is usually quite safe, but a seepage that is discoloured by the wall material should be considered a danger to the wall itself. Raking or harrowing of the side of the wall in the water of the dam is usually the best means of sealing this type of seepage."

Chapter 8 (Trees)
  "Keyline timber clearing is planned to derive the greatest benefit from trees for the whole of the farm.
  First, trees are left in strips or belts wide enough to keep some semblance of forest conditions in the timber for its normal healthy growth.
  Steep country is not left in full timber, but partially cleared and timber strips are left to serve as wind protection for the property.
  The Keyline is again the planning guide for clearing. The first timber strip half to one chain wide is left just below the Keyline and forms a Keyline Timber Strip.
  In most areas the lower side of this timber strip is suitable for a farm road being drier generally than the land above the timber strip. Crop or pasture suffers more from the effects of moisture lost to the trees on the lower side of a timber strip. However, when a road follows along the lower side of the strip the little extra water run-off from the road causes both grass and crop to grow well right up to the road."

" By clearing the steep country on this pattern, more and better grass areas are available and better timber will grow in the timber strips.
  Very short steep slope country is always of greater value when cleared and Keyline developed. Suitable timber strips are left on the flatter top country above."

" If each paddock in turn is closed to stock and cropped for two years or more in each ten or twelve years, young trees develop in the timber strips and permanency of timber belts is secured.
  To sum up the simple plan of Keyline timber clearing:
  Decide on the location of the largest paddock areas--see further comment in the chapter headed "The Plan"--and locate the Keyline or Common Keyline of this section. Then peg or suitably mark a strip or belt from 30 feet to 60 feet wide parallel to the Keyline below it. This belt is to remain in timber."