I have added a new video to my YouTube channel HERE.
I find that so many golfers have limited hip internal rotation (as well as general limited hip mobility). Hip internal rotation is important during the golf swing for a couple of reasons. Firstly, it can effect movement during the golf swing, showing up in certain swing characteristics (such as a sway or slide) and it could increase your risk of low back injury.
Furthermore, the hips are a massive power generator in the golf swing, and if they are not functioning properly, limited hip mobility could rob you of distance off the tee.