Let's now introduce the SPC concept with reference to the pelvis. I personally believe it is the most crucial area of a human body when comes both to creating power as well as maintaining balance of the motion.
The pelvis is a multibone solid structure that links the upper body with the lower body. It is so important because our lower body consists of two limbs, therefore, the pelvis contains two very large joints called hip joints. The problems created by the pelvis motion, especially in rotary types of motion, are countless. The same is with the golf swing motion. Improper motion of the pelvis area can be the most often appearing culprit for inconsistency and lack of repeatability.
Generally, the lead hip downswing motion contains mainly the rotational element while the motion of the rear hip contains mainly the linear one. This simplified idea matches ideally the SPC concept since the pelvis cannot be "compressed" any other way. Imagine now that the rear hip tries to take the impact position of the lead hip that, at transition, is North-West of it. Since the vertical axis of rotation goes through the lead hip - we are bipedals and that is why the most efficient rotation happens if the vertical axis of rotation is being transferred to one of the two hip joints. Having said that, I would like anyhow to stress that the CoG transfer is a more complex thing than just transferring 100% of the weight/pressure to the lead hip. It should be done neither too early not too late. This is one of the most important timing issues in the motion that can be, I do hope, excellently eliminated via the SPC concept.
OK, as it has been already discussed when talking about the spine, the rear side adduction that happens as a natural consequence of a correctly performed backswing is also the crucial force in the pelvis area. Before the lead side leads the parade via its abduction the sagittal plane of the pelvis should be compressed. It won't be if we start the transition by lead hip pulling as it was drawn in '5 Lessons' (famous elastic tape image), we need to let the rear hip adduct. If I were Mr.Ravielli, I'd draw a second, much thinner elastic tape that comes throughout the body to the rear hip to make all golfers be conscious about the rear side of the body working.
It all can happen only when the rear hip motion is being restricted only via finding natural limitation (thanks e.g. to rear knee joint preset). As we already know the lumbar part of the spine (as well as cervical one) will tend to set the direction of the movement to South-West and the rear hip moves linearily to the West trying to "compress" the pelvis in search of creating theoretical one common joint of the two, through which the vertical axis of rotation will run. It feels like we still let the lead side be pulled right when the rear side has already finished its abduction and starts to reverse the motion, i.e. starts to adduct. Moreover, it is a trampolline effect that accelerates the lead hip rotation. Lastly, the SPC will take care of a fluent and proper CoG transfer to the lead hip joint - without it timing issues can kill the swing. With it - we will have a decent degree of automatism in the motion of this area and we can let a powerful transition and downswing happen by itself. Of course, it all cannot happen without a correct legs/feets work and correct presetted stance.
Coming to the SPC of the uder-pelvis section we have to be very aware how the concept works in the hips area described earlier, especially what is the motion and function of the rear hip joint. Its linear movement should determine our stance as well as the motion of the rear knee joint.
The crucial thing is to understand that the rear hip joint is turning at the backswing phase with the smallest possible displacement in the coronal plane, i.e. on the North-South axis. The coronal plane is this one that dissect the body for rear and front halves. It is the lead hip joint that is moving on the circle in this phase as well as later during the downswing. There should be no change between those two phases since it would only create an unnecessary disorder in the fluency of the motion that must run in 180* opposite directions.
It matches perfectly the scenario of the SPC of both upper as well as pelvis area of the human body. The rear knee joint should be more South than the lead one at the top, so that it could move sort of behind the lead knee joint at transition. Of course, it is an almost invisible motion that compresses the plane between legs that, similarily to other areas of the body, is caused because of finding a natural limitation in its motion. It looks like, during a very short period of time, the distance between both joints decreases in a saggital plane while it remains the same in the coronal plane. Just imagine scissors.
Feet now - personally, I can see no particular reason why the stance should not help to perform the above described action. Moreover, I can finally understand the diagonal stance Mr.Hogan performed while hitting longer clubs and I am surprised he did not use it with his short clubs as well. Or better said, he did not described it in the book but the concept of the diagonal stance was often visible when he even hit wedges. Some Hogan theorists even claimed that his feet stance was always closed to the target line - only his lead foot was in a flared position that gave the illusion of a slightly open stance. In fact, Hogan's hips were in an open position in relation to his feet at his stance.
Please look for details in the posts dedicated to the Diagonal Stance.
