Chest and hips.

After your hip height passes below your knees, you will feel the stretch in your legs and hips reach such a peak that you cannot go any lower without rounding your back. As soon as you feel this ‘bounce’ pushing you back up, drive your feet into the floor and your traps into the bar so that your hips and chest rise together.
Stretch Reflex
At the bottom of a full squat... several muscle groups reach a full stretch [which] will produce a contraction that feels like a natural bounce out of the bottom - a stretch reflex. Do not dive bomb your squat and bounce your hamstrings off your calves... The lower back should remain rigid throughout the entire squat. Maintaining tension throughout the entire squat is hard and it's uncomfortable but it's absolutely necessary.
Alan Thrall
I actually never even think about coming back up. I'm so tight I'm just like a spring. I just start to come back up, now I finished the movement by squeezing my glutes and driving them forward.
Super Training
Squat down, break parallel, then quickly reverse the movement by Squatting back up. The weight will be easier to Squat because you’ll use the Stretch Reflex. Your leg muscles will contract harder because the way down stretched them. The harder your muscles contract, the stronger you are on the way up. The key is to maintain proper form at the bottom by keeping your knees out, hips back and lower back neutral.
StrongLifts generally don't want to pause in the hole at all, but rather take advantage of the stretch reflex at the bottom and immediately reverse the motion the second you hit depth.
Jeff Nippard
Feet into floor + traps into bar
Squat University
...initiate the ascent by driving your traps back into the bar aggressively while driving your feet through the floor... Since the bar is trying to crush you forward (imposing a spinal flexion and hip flexion moment), driving your traps back into the bar will counter that tendency...continue driving your traps back into the bar aggressively [through the sticking point], while simultaneously trying to drive your hips under the bar. Your knees will probably shift forward as well.
Stronger By Science
The squat isn't just pushing with your legs, you have to use your back at the same time. So as you push up off the floor you want to drive your back up into the bar as hard as you can and pop your chest up...You don't want to come backward... so you'll push up with your lower body and you're going to take the chin and chest and drive it straight up through. That drives the bar up [vertically] and not back.
Super Training
Keep your chest up out of the hole by driving your head and shoulder back into the bar. Also drive your elbows forward under the bar (again there may not be much actual movement of the elbows but the intent will help you avoid collapsing forward) as you initiate the drive out of the hole.
As you’re coming up out of the hole, don’t think about standing up – think about pushing BACK into the bar. This will help you keep your chest up and the bar moving in a straight line.
Robertson Training Systems
We're watching for his butt to come up and the barbell to come up at the same speed. The knees drive the hips up and because he's rigid in his mid-back, the bar comes with it.
Barbell Logic
Shoulders should be the first thing to move coming back up... out of the hole... like you're trying to heave that bar off of your back and throw it into the back wall... Come out of the bottom with your shoulders first, chin first... some people will say chest first.
Elite FTS
We're gonna take that head and shoulders unit - which again is locked in like she's wearing a neck brace - and we're gonna drive it all back up into the bar... and what she's trying to do is create even pressure with her feet into the floor and her upper back into the bar to maximize both the strength of our legs and lower back as we lift the weight.