When squatting, whatever happens in the bottom position is important. Not only do you need to keep the shape of your back, you also need to know how to catch the bounce. This allows you to utilize the stretching of your quads during the eccentric phase (lowering down to the bottom position) to help you activate that stretch reflex to give you elastic energy to come back up. The slower your downward movement of the squat is, the less elastic energy will you have to facilitate this. On the flip side, some people know how to catch the bounce but the moment they rebound back up, they lose the shape of the back (I'm guilty of that myself sometimes but I'm working on it). So if you are able to get a grasp of how to bounce out of the bottom position, your main concern would be to maintain your torso's shape.
I have also discovered that it is a lifting rhythm that you need to get into. Learning to time the bottom position and pushing back up at the right time is something that takes alot of practice. So keep squatting and work on that aspect of your technique!
P.S. Seeing so many people trying to understand this part of the squat, I will be putting up something about squatting in the coming weeks! So stay tuned for that. Cheers.
Stay Strong and Keep on Squatting,
The Training Geek.