Important: Not only must you understand weight shifting, but you must understand stopping the weight shift - ending up "propped" on the ground leg. Many beginners 'shift through' their balance. Their kicks resemble a swinging exercise.
Basic Set-up: Viewed from the side, there is a point in time when you're vertical, and standing one one leg. At this time, the shoulder of the opposite side to the leg is vertically above the foot. Viewed from the front, the same relationship applies. You don't 'stand around' in this position, but you go this far. The middle of the pelvic bowl is vertically between the foot and opposite shoulder - ie, the top doesn't lean. I call this shoulder point balance kicking. It is pretty theatrical but is a good advancement in co-ordination.
Advanced Set-up: Internally, you feel like you've gone through the same vertical standing point, but externally, you don't show it (ie, don't telegraph). The front torso between dan jun and solar plexus are most active in storing kick power during the transition from two legs to 'chamber'. The storages are both torsional and concave/convex. This I call internal (or centre) point balance kicking, and is levels above shoulder point balance kicking - in power, fighting utility and health.
Internal Aspect: The same principles can be given in terms of the three dan juns (Chinese: dan tien). The upper dan jun is the third eye (located just above the mid-point of the brow, and about two centimeter inside). This point may be visualised as being 3 to 4 centimeters in diameter. The middle dan jun is in the region of the solar plexus (just below the xiphoid process) - again about 2 centimeters inside, with a diameter of maybe 6 centimeters. The lower (and most familiar) dan jun is located about 4 cm below the navel and about one third towards the spine. It's size can be taken as about 6 cm as well, but may be moderated by breath (this is a big topic which will receive attention later).
Another key 'point' (on the Governor Vessel - spine) is the Myung Moon (Chinese: Ming Men), which is between the second and third lumbar vertabrae. It is 'as wide as a verebra' - ie about 5 centimeters. It's about 6 centimeters inside, too. Rotation of the shoulder-line relative to the pelvis is centred on the Myung Moon. The Eastern notion of 'waist' (loose and flexible waist, power from the waist) is basically about the Myung Moon, not the belly. Rotations and stores centred on the Myung Moon are at the same time smooth, and powerful (train hard to find this).
The initial movements (weight transfer, lift heel, kink knee) amount to a displacement of the middle dan jun, with the upper dan jun smoothly conforming to the same movement. The lower dan jun follows this movement - so that some storage of 'tension' occurs in and 'across' the region between dan juns, along several direction. This includes some relative shoulder to hip rotation (maybe about to 15 degrees). This kind of co-ordination, [ie, various shoulder-to-hip tensions and mutual support (eg, right shoulder approaches left hip, or, side of right shoulder to side of right hip, etc)], and ground strength from the supporting foot provide much of the 'whole body' power.
This is also the start of the 'chambering' motion. [Bob Duggan refers to a stress reflex, which seems to be the same animal]. From here, the familiar chambering takes place - with store and balance - where the other so far unused co-ordinations are factored in to the kick.
The upper body never loses an easy functional relationship to the kicking leg (or the ground leg). Some energy was stored above the pelvis. You wish to direct it through the pelvis and thigh to impact.
Then comes the throw of the kick (either thrust or snap), and the extent of loading, focus, whip, etc, which applies to that kick and target. Really strong kicks transfer power from the ground leg and the energy in the middle dan jun, into the target. This kind of kicking is not advised for beginners or people who are out of condition.
These notions are hard to come to terms with, but are extremely useful in developing powerful kicks with quickness of delivery and recovery. They must be drilled till the body feelings are automatic. Then they must be drilled, in combinations, using held targets. Full power kicks also need practice on heavy targets (eg, punching bags - starting from a 26 inch (long) bag).
I'll be adding a page on Performance Technique (which is about how you might kick, strike, etc, ... for a demonstration or movie), and another one about Internal Power Generation for Kicking (which is about the whole-body approach to kicking, using principles of loading, focussed release and sudden power).
Using the ground, and momentary 'lockout' to let all power into the target is a fairly advanced method of kicking. If you really can do this, you can do kicks in slow motion which hit large targets solidly.
Even more advanced methods utilise a whipping principle, rather than a momentary 'lock out'. Many beginners use 'whippy kicks', but these lack the delivery of strong power from a loose (but not floppy) body. Train for effective lock out, first.