Very interesting and timely for me, given I'm working on this - the position of the right elbow during transition and first half of downswing.
I used to try to move my right elbow in front of my hip while turning the hips to start the downswing. That caused me to get the club stuck behind me by thrusting the hands away from the body too much ( and below ideal plane) and forced active hand action to square the face...resulting in a LOT of nasty hooks. Now I'm just trying to get the right elbow alongside the hip instead of in front of it, trying to keep my hands closer to my body, and the club not so flat coming down, and its working very well now. No more hooks and the ball starts out on a real good path. I'm still gooving it on my P3ProSwing and at the range, so its still a work in progress, but I believe it is working.
So the moral of the story is dont let that elbow get so far in front of the hip that it forces your hands too far away from the body - big trouble there !!!