Let me quote my man Rip on this:
"Bounce occurs optimally at the correct speed of descent. If your descent is too fast, the bounce will be less effective, and much less safe, because the only way to drop too fast is to relax something. Muscles tightened in the squat descent store elastic energy; tight muscles also keep your back, hips, and knees in the correct, safe positions. If you are loose enough to drop into the bottom of the squat much faster than you can come up, you need to tighten up more – and it may help to think of this as slowing – on the way down. A loose descent can allow joints to be jammed into positions they should not occupy, and this is how most people get hurt squatting: getting out of good position by going down so fast that they cannot maintain proper technique. This may be how squats got an undeserved bad reputation. Don’t contribute to the problem by dive-bombing into the bottom.
Rippetoe, Mark (2013-11-07). Starting Strength (Kindle Locations 1337-1341). The Aasgaard Company. Kindle Edition."
Also watch Chris Duffin's vids where he talks about ripping the floor apart with your feet so your quads and glutes are activated from the start rather than suddenly having to jump into action at the bottom of the squat.