[EdZ]

I am a swinger who has a recurring issue with my hands dropping under plane in the transition, resulting in fat shots, heel contact and a lack of compression. It seems logical that I am losing extensor action at the top of my swing. Any feels or keys that would help me start down on plane? When I concentrate on lower body starting the downswing, the flattening worsens significantly.

It sounds like you are either already well inside/flat going back, or you slide/move well forward before impact.

Can you make slow motion practice swings with your backside up against a wall without hitting it? (a foot or two away to start)

This drill will give you a much more 'up and down' feel, and help you be a bit more stable/not slide. You may be 'lifting and rolling' in the backswing as well - work on feeling your hands moving straight back while keeping the clubhead outside the hands until hip high. This goes well with the 'karate chop' feel of the left hand going more 'up and down'.