Strong quads and adductors are needed here. The bottom phase is quite literally the bottom half of the movement, from the 'bounce' out of the hole up to the point where the hips are just above the knees.

The squat can be divided into roughly two movement phases: the bottom and the top phase. And those sticking points are combated quite easily with the right selection of exercises. Lucky for us though, these differences typically result in quite predictable sticking points within the squat. Both of these techniques are totally fine, but they result in vastly different muscular (and strength) development from what is considered to be exactly the same exercise. For those with short legs and a longer torso on the other hand, an upright torso position is the way to be stable and strong. Do you have long legs and a short torso? Then you probably have to lean forward quite a bit when squatting just to keep the center of gravity over your feet and not fall on your ass. If there is one movement that is massively influenced by individual body-type, it’s the squat.