DG,
I disagree with some of SMK's comments - my replies are in italics.
1. (4:26 - 6:03 ) - Example of 'nose flicking' to explain the 'Stretch Shorten Cycle' for creating a greater 'starting' force in the abdominal muscles in the downswing.
I agree that if one flicks the index finger by preventing it from flicking in the initial phase can increase the speed of finger flicking when the impedance is suddenly released - because the muscles are maximally contracting before one allows the index finger to snap-straighten. However, that's not what is happening when one generates dynamic X-factor by starting the pelvic motion first. Under those conditions, the abdominal muscles are being passively stretched when they are not yet contracting. 2. (6:28 - 7:40) - The more separation between the COP and your mass (I am assuming he means the COM), the more potential there is to add speed into the system. You can get more speed if you have 2 legs on the ground because you're more able to shift your pressure around relative to your COM. If you only have one leg on the ground, you can't get that separation that's going to inhibit that speed.
I don't agree because one is not primarily using changes in COP to rotate the body and also the body mass (COM) is not necessarily moving sideways away from its central position. I think that most of the body's rotation is due to muscular forces.
3. (9:44- 14:30) - Disadvantages/Advantages of trail knee flex
SMK argues that allowing the right knee to straighten can increase pelvic rotation and that can increase the length of the hand arc path. I disagree! Most of the length of the hand arc path in golfers who have a long hand arc path is due to the left arm operating in a more vertical plane - by having a steeper left arm angle (eg. Jamie Sadlowski). Pelvic rotation operates in a more horizontal plane, and I do not believe that it necessarily increases the length of the hand arc path. Also, it is possible that the speed of counterclockwise rotation of the pelvis will be decreased during the early downswing if the right knee straightens more between P1 => P4, and that may translate to a slower "average" speed of hand motion down the hand arc path.
4. (17:25 - 24:21) -Disadvantages/Advantages of lead wrist flexion & trail hand function. There is an interesting bit at 23:30-24:21 relating to weaker grips where SMK claims how important it is get the lead wrist into flexion to get speed into the club.
I disagree with SMK's reasoning. He referred to Webb Simpson who has a lot of lead wrist extension at P4. However, WS uses a very strong lead hand grip strength and he does not need to flex his lead wrist during the downswing to close his clubface relative to his hand arc path. If he changes to having a more flexed lead wrist during his downswing, then that will mainly affect his accumulator #3 angle because the back of his lead wrist is parallel to the ball-target line between P6 => P7. SMK's argument that bowing the lead wrist in golfers who use a weak lead hand grip can increase clubhead speed because the lead wrist will be moving towards lead wrist extension through impact makes no sense - because those weak lead hand grip strength golfers (eg. Collin Morikawa, Jon Rahm and Jordan Spieth) maintain a bowed lead wrist between P7 => P7.2 and they do not rapidly-and-massively extend their lead wrist through impact.
I also don't agree that changes in right wrist extension will affect clubhead speed or the efficacy of the release of PA#3 because of the confounding variable of the degree of "running-out-of-right arm" phenomenon.
5. (27:38 -31:14) - Ground forces to assist rotation . There is some discussion about LPGA players where their ground forces do not reflect their increased hip speed rotation compared to PGA players.
SMK claims that LPGA golfers can rotate their pelvis fast without generating high horizontal GRM values. That claim is probably true. However, where is the "evidence" that generating higher horizontal GRM's correlates with a more efficacious overall pivot motion (involving both the pelvis and upper torso) in male pro golfers.? I suspect that the efficacy of the muscular contraction of the pelvic girdle and abdominal muscles play a more important role!
6. (31:14 -33:30) - Knee flexion and how retaining trail knee flex in the backswing/transition might tend to cause a right leg launch and hip spin. Having lead knee flex increase approaching the end of the backswing and the start of the downswing to get a counter movement stretch in the muscles for the prepared extension of the lead knee to help rotate the pelvis and increase vertical ground force.
I think that SMK's argument is irrational when he implies that allowing the lead knee to flex more in the backswing can induce a stretch-shorten cycle in the lead leg that will increase the efficacy of the lead leg straightening action that happens in the downswing - because the lead leg should only straighten after P5. Between P4 => P5, during the Sam Snead "sit-down" maneuver's hip-squaring phase, both knees are equally flexed.
Jeff.