Actually, it is a very simple phenomenon that allows to release the whole kinetic chain in a proper way and is a very important part of the SPC concept called the second (impact) compression.
In order to understand it well we must differ the two scenarios - a. compression phase that is aimed at the change of orientation (direction of movement) and b. compression phase that happens without the change of orientation (direction of movement). The first one of course happens between backswing and downswing, while the second one happens at impact when the whole kinetic chain is being released at the ball. The chain is the whole human body starting from the ground/feet ending on the arms/hands/clubhead. The release is just passing the stored energy to the more distal parts of the system and it is done via consequent stalling of former links in this chain - i.e. in order to pass the energy to the arms, the main body and the shoulder joint must stall.
But during downswing nothing can stall because the body is in constant rapid movement till the finish. So how to pass the energy to the arms ? Look at this photo sequence:
The ball position indicates vertically when the chain is being released correctly. It means that the lead shoulder should (hypothetically) stall at this line in order to let the distal limb and, consequently, the clubshaft and clubhead to release the energy and achieve the peak velocity. If the lead shoulder continues to move linearily to the target in a 2-D, the chain wouldn't be released at all.
However, if the lead shoulder, which must be still in rapid motion, stops to move horizontally and starts to move vertically up (say, on the red line up) in 3-D, so the chain must be released because its position in relation to the ball does not change in the horizontal 2-D sense - say, the ball "sees" the lead shoulder stalls completely.
So, as we can see, it is really a very simple physical phenomenon that allows the compression phase to occur without stopping the motion and without changing the orientation of the whole movement. Moreover, it guarantees automatically that the angular velocity of the clubhead (as the most distal part of the system) increases as per parametric acceleration phenomenon. The second (impact) compression phase:
Otherwise -- how one could start the motion without conscious mind engagement ?
After the trigger compression, the rear side uses the created momentum (say, as bouncing from a rubber wall) and the whole rear side takes the momentum and starts naturally to pull up, around and back creating torques in the joints from the ground up. Using the TC phase practically will leave no option for a golfer but utilizing the initially built momentum via the active rear side in the backswing. The rest is just a chain of unintentional events that everyone should dream of.