It is very important to underline that the crucial thing when seeing the motion through compression and expansion elements is to understand that the expansion elements are framed within compression ones. I mean there is no expansion that does not begin and end with a compression. That's why the working name of the concept is the Sagittal Plane Compression and not the Sagittal Plane Expansion.
This concept reflects a perfect example of creating and using the kinetic energy in the swing motion. The compression allows to store energy and pass it to the passive side of the body sequentially from the ground up which can happen only when the orientation of the motion corresponds with the active side of the moving body.
Compression is being controlled by the natural limitations of the human body based on stable skeleton/joints structure. Muscles that are generators of the energy cannot be used to control - they must be controlled by something stable. The joints won't let the muscles waste the energy while they find their natural limitations during the torquing process. Moreover, they will allow to react into the opposite direction using the same energy since the limitation is one-sided only - as e.g. during the transition providded there is a weight shift to the rear side with preset rear leg joints (bouncing wall effect).
Expansion, as a process always enframed between two compression phases, is in fact being controlled by those compression phases of the lower located parts of the body (vide: transverse planes). I know how silly it sounds but actually this is what happens. It won't sound silly if we take into account that the compression/expansion phases occur sequentially in time from the ground up. For instance - the transition - when the legs have finished their transition compression and are already in the expansion phase, the pelvis actually is in the middle of its transition compression and the upper body is still in the backswing expansion phase. The chain of events works by itself almost automatically then. Say, the expansion phase of the body part that is located higher meets the compression phase of the lower located one which won't let another scenario to happen unintentionally.
What is also of big importance -- pulling vs. pushing issue. Each compression phase requires taking the momentum over by the other sagittal part of the body fluently. It can be done only via pulling from the core out. However, the smaller is the gap to the next compression phase (or by the end of the expansion phase) the more pushing instead pulling occurs -- as pulling converts into pushing with the momentum already rocked on. The leading side starts to push it as pulling would guarantee no further acceleration.
Observe how Mr.Hogan builds the frim rear side sequentially from the ground up thanks to the rear ankle and knee joints preset. Observe how his rear hip reacts unintentionally to the sequentially built torques and moves linearily. Observe how Mr.Hogan can ensure automatically that the downswing is a pure undisturbed rotation as a natural consequence of what has just happened before in the motion or even during the setup.
Mr.Hogan's whole swing is a beautiful compilation of compression and expansion phases that occur in a proper sequence from the ground up thanks to properly built torques in the joints.
As we know it is called a Sagittal Plane Compression (SPC) concept:
How the firm rear side can be achieved automatically in practice ? Observe closely Mr.Hogan's rear feet action at setup in this video:
The action contains a clockwise turn of the rear ankle and knee joints without changing the rear foot square to the target line position till the limit. Turning the rear heel outwards first (this makes the foot perpendicular to the target) and then turning out both joints. Like one wants to bring your rear heel inwards again, which won't happen because of shear forces between the foot and the ground.
One should imagine that when standing on ice your rear foot would spin outwards because shear forces are practically not present. Since there are a lot of shear forces between one's foot (armed with spikes) and relatively soft ground, a substantial amount of torque is being built. One will feel the torque. One will also see that your rear knee joint bends slightly inwards as a consequence of this action.
It is one of the most importants little secrets of Mr.Hogan aimed at building the firm rear side from the ground up that, consequently, leads to automation of the whole motion.
The arrows show the torque vectors while the blue slanted line shows the natural reaction of the knee joint to the process of its presetting. The bending inward is an additional help when building the firm rear side from the ground up.
