We got an exercise from Zygmunt that I feel could benefit a lot of people. We call them "No hands no feet snatches". You perform a snatch, but with no hook grip and with the intention of not coming off the floor and not allowing any foot movement. Now, a bit of heel rise is ok, but the ball of the foot should not come up and the foot should not move.
The lack of a hook grip eliminates the use of any "jerkyness" and assures smooth acceleration during the pull, and the lack of foot movement and intent to keep the ball of the foot planted on the ground means you have to start pulling under IMMEDIATELY after extension, or even a bit before full extension. The result is a smooth pull and a FAST descent under the bar. This exercise has a very positive influence on the snatches of some athletes here, and I encourage everyone to try it. We now do it twice a week, once heavy, and once as a warmup for regular snatching.
Below are Jon North and Donny Shankle performing this exercise. Jon is doing 132 kilos, a new record for him, and you can see his right foot just barely slide, which is not supposed to happen. Donny is doing 110 kilos, you can see that his heals rise, but the ball of his foot remails firmly planted without coming up and without moving at all. His maximum in the exersize is 137 kilos.