The EEP (early elbow plane) is a logical result of benefitting from one of the golden rules of (bio)physics concerning the perpendicularily of the distal parts motion to the spine (core).
The EEP is a description of a biophysical phenomenon when a classic double shifter achieves the EP relatively very early in the downswing with the shaft bissecting the forearm; a decent amount of lag is necessary to perform the EEP. An EEP golfer receives full support from both rear humerus (in relation to the body alongside with which is moving) as well as from rear forearm that supports the shaft and clubhead until impact being in-line with it and at a perpendicular angle to the core.
The best example is post-accident Hogan who achieves the EEP very early in the downswing (his rear elbow moving forward early and fast after transition) until shaft becomes in-line with rear forearm and rear forearm perpendicular to his core. From this moment there is his turning main body only, the rest is in status quo in relation to it which may be certainly considered as the best possible biomotoric scenario for consistency in the downswing phase:
As we already know, the backswing must be lead by the rear side, for the reasons mentioned earlier. It won't let a too inside takeaway which is the main enemy of flattening the shaft plane after transition = one of the biggest enemies of the downswing EEP. The best results are achieved if it is being performed sequentially from the ground up at ca. 45* angle up & back. The downswing phase occurs sequentially from the ground up as well thanks to an automated transition that is possible thanks to finding limitations in the joints sequentially from the ground up. The overtorques in the joints cause the trampoline effect when changing the orientation of the motion (from backswing to downswing). The lower the body part the sooner it finishes backswing and the sooner it starts downswing. It creates optimal condition for the plane to drop (congruently) from TSP to EP as soon as possible.
