The "push off" topic is one of great debate. I'm of the opinion that there is a lateral push against the ground/rubber to initiate movement down the hill. This lateral push is itself initiated in the hip. After that, there is no push in the quads - as that back leg and foot turn over you'll see that the back knee maintains a bend. There may be a small push towards the end of the stride via plantar flexion (i.e. extending the ankle and pushing off with the toes).
Positioning on the rubber can affect posture and balance. As a pitcher throws to a target, his shoulders will attempt to square up to the target. If the pitcher has tracked in a direction different from the target, the attempt to square up may result in a posture shift (i.e. lean) late in the delivery. Late posture shifts will pull the release point back and can make it inconsistent. (As a pitching instructor, I use position moves as fixes for posture issues.) Posture shifts can also lead to early shoulder rotation which can be a health issue as much as a performance issue. So, while holes in front of the rubber are certainly an issue, be aware of the issues you can create by changing position on the rubber.
Angle of the foot relative to the rubber can also affect shoulder position at release point so be careful about that too. In my opinion, trying to get more push off isn't worth ending up over-rotated at release.