I agree that it is the opening of his shoulders that seems to be the main cause of the
ball ending up behind his back. However, when he is in his stance the
ball is in front of his right leg and is not visible. When he pushes the
ball straight out from his leg the
ball becomes visible and seems to initially swing in a left to right direction until reaching his leg when the opening of the shoulders causes the
ball to move in a right to left direction giving the impression that the
ball is moving around his leg. It is this sudden change in direction in his backswing that seems to result in a worst case scenario of swinging behind the back. (Ex. the frame where he shoots the 7-pin where he really wrapped his arm behind his back.) He is inconsistent as to how he pushes the
ball out. When he is not pushing the
ball straight out and going around his leg, he is pushing the
ball out slightly to the right which allows the
ball to clear his leg in a straight swing before he opens his shoulders. This cuts the swinging behind the back by half but he is still losing leverage at the foul line because the backswing is not close to his body and the
ball is too far from his ankle at the release point. He has to devise a way to keep his swing close to his body while keeping a staight armswing.
Simply put, when in his stance the
ball should be ideally over his right foot with the left edge of the
ball over the right edge of his left foot. During his backswing the trick is to move the
ball past the right leg while keeping the swing straight. This is what he should be working on. The
tape on the floor exercise was a suggestion to achieve the latter.