I think it's worth saying that they are advanced techniques that have a decent potential for injury. Inexperienced people should avoid them entirely. Even experienced lifters should have someone trained watching them as a spotter. Remember, your back needs to be arched for the entire set. Anything else is likely to cause injury.

I very much disagree about bouncing out of a squat. Maybe while warming up or with light weight it's ok, but with heavy weights, the safest technique is a steady and smooth motion while staring at the top of the rack to force a straight back.