[BerntR]

If the left arm was hinged to the pivot at the swing center (as it is in figure 1L) it would only be able to deliver radial force to the primary assembly, i.e. keep the primary lever assembly rotating.

But the left arm is hinged ahead of the swing center and therefore able to pull the primary lever assembly forward and not just around. If you feel any reacton from the club in the left arm, the left arm is tragging the club - not only in circles but also forward. The release of the primary lever assembly is a powered release. It does the same kind of work as the right side, only in a different manner.

The endless belt, ref 2k shows linear travel versus radial travel. That is not at all the same as linear versus radial force.

If the endless belt is moving at constant speed, the club will also have a constant speed during it's linear travel. That means that the club is not subject to any forces at all. But matter changes when it takes the curve. The change of direction is caused by a radial force. This force doesn't increase the speed; it only changes the direction.

If no linear force is applied by the belt on the club, the butt end of the club will slow down and the club face end will speed up. So the velocity energy in the lever is redistributed. (Correspondingly, the hands slow down as the clubhead picks up speed.) But the total mass-velocity of the endless belt-club will remain constant.

If the club speed is increased and not just redistributed, it's due to work from a linear force. (Linear force in the TGM sense; A force that works in the same direction as the club at any moment; in other words a tangential force).

The left shoulder pull does both those things. It can't do one without the other.