I've seen a lot of variations but this is how we do it at All Out Effort for it to be considered proper technique.
1. Chest must hit the ground.
2. Must jump up at the end.
3. Feet must be on the outside of your hands at all times.
We had a little contest at the gym and Young who is one of our clients, banged out 71 in 5 minutes at the end of this workout. All with proper form.