This concept, that I call now Sagittal Plane Compression (in fact, the first working title was Sagittal Axis Twist but I disliked this because it was misleading a bit), refers to a lot of dynamic aspects that were not automated well in the whole Big Picture...such as CoG transfer at transition, proper pelvis area work, maintaining the CoG properly during downswing until impact and even influencing a proper knee bend angle. In short, it can be a true missing link in the theory.
First, what is the sagittal plane:
Shortly, it is a virtual imaginary plane that divides the whole body into left and right portions. The wide neurologic aspect that we are omitting now concerns human brain halves and their interactions. What we want to concentrate is only the physical actions of lead and rear body parts in the swing. There are two main notions that are associated with the sagittal plane - abduction and adduction.
Abduction is a movement which drives a part of the body away from the sagittal plane of the body and, therefore, it is opposed to adduction that is a movement which brings a given part of the body closer to the sagittal plane.
Long story short, I have concluded that the backswing phase can be mainly described as the abduction of the rear side (of course, it is simultaneously the adducton of the lead side) while the downswing is mainly the abduction of the lead side (of course, it is simultaneously the adducton of the rear side). I have categorized it this way since this is a real importance of motion in a golf swing while having the target on the lead side. It also matches pretty well the theory of Small X-Factors as well as the pulling force of the rear side during backswing as the most natural and efficient one (presented in details in The Main Body section). So - abduction of the rear side leads backswing - it contains not only main body pulling action but also rear arm and elbow joint pulling/folding action. Let's forget for the moment about lead side work, such as lead forearm rotation that happens in a completely different plane.
The downswing, OTOH, is mainly the abduction of the lead side - the main body pulling action and the lead hip action. Let's forget here about the magic of the rear forearm as a support to the above.
The crucial part in the whole SPC concept is dynamic interaction between those two abductions. As we know, the most consistent ballstrikers always started the downswing before the backswing ended, if I may simplify the phenomenon. It means in reality that the sagital plane must be "compressed" or "twisted" during the transition, creating a sort of a trampolline for the lead side of the body abduction.
Going further, it implicates that the rear side of the body must start its adduction before the lead side starts to act. It is not difficult a task at all because we can create necessary limitations in certain areas, such as in rear leg, rear eye or grip, etc. (all of them are described in those five sections of the thread). The SPC start to happen when the lead side inertia is being stopped by the rear side due to those limitations, but what is more crucial is that this effect is somehow doubled while the rear side starts to adduct.
The whole problem is to find a chain in the whole action looking at both sides of the body INDEPENDENTLY, and to find areas where the SPC might work (we need to remember that the base of the body, i.e. bones are hardly compressible). It would be necessary to "cheat" the skeleton.
I'd like to throw a trial explanation of the spine rotation influences and helps in automating the transition. The evolution shaped human spine in curves. The major part of the spine (thoracic part) is flexed while the two distal smaller parts (cervical and lumbar-sacral) are extended. Ironically, the thoracic part, that is braced with ribs for the most part is the part that rotates much more than the other ones. Why ? During the backswing, the cervical part rotation is being limited by the head movement that is limited by the eye sight range; the lumbar-sacral part rotation is limited by the very pelvis as well as by rear leg preset action at the setup.
The bigger amount of rotation of the middle part causes both distal parts to react accordingly when the small X-factors are being achieved. They both extend more than in a stationary mode in relation to the mid part; in fact, we can also state that the thoracic part flexes a bit more as well while turning back in relation to the other two sections.
This causes both the cervical as well as lumbar part of the spine go up and back (precisely in this order since the rotation of the shoulder area, i.e. upper part of the thoracic section is the biggest); when the backswing is complated correctly, a slight but noticeable shift towards South-West of the top of the spine can be observed; same happens a fraction later with the lumbar part that golf instructors often call informally as tailbone. South-West direction is exactly the direction of the perfect transition move that leads to a correct hip movement at the downswing in the rotary swing.
As a side note - the lower is the plane the easier is to find the limitation of the main body during backswing - high two plane golfers seldom find the maximum amount of the small X-factor of the shoulder girdle just because of the arms working on a higher plane.
