Mills has a big hang-up with the towel drill. I think it's his vehicle for taking stabs at Tom House. But, really, he needs to get over it. I also don't like his demonstration of the drill because when he shows how the drill supposedly leads to cartwheeling instead of rotating, his demonstration of rotation shows the hips and shoulders rotating together. I think he knows better but he sure didn't show it in that video.
House does not teach the purpose of the towel drill being for "extension". In fact, House teaches that the release point is the result of the mechanics leading up to it - not a conscious effort of the pitcher to extend his arm out front. Mills misses the point on this.
What I noticed in Bower's video is a lack of trunk flexion. Normally, after the shoulders rotate and right before the arm whips forward, the upper torso will be in a stacked position with the low back arched in an isometric load and then the low back releases and the trunk flexes forward. In Bower's case, however, he stays upright. So, he's missing the last link in the chain firing right before the arm whips forward. This also is likely to make the follow-through more stressful.
So, I'm with Hose - I think the towel drill would be a great way to work on incorporating trunk flexion into the delivery.