I’m not an expert, but here’s my opinion.
There are two basic ways people squat. One way is keeping the bar midway down the traps, and starting the movement by pushing your butt backwards therefore “breaking” with your hips. You keep an arch in your back (meaning your spine does not bend forward except at the hips) and at the bottom position your hips should be at about the same height as your knees, meaning you’ve squatted “to parallel”. Your back will be at an angle to the ground, but not rounded. Here you are squatting using a lot of muscles from the back side of your body - hamstrings, back, etc.
The other way, you keep the bar higher up on your shoulders, and rather than pushing your butt back you start by bending your knees. With this technique your back is more perpendicular to the ground as your descend, and your knees come more foreward over your feet. You also want to be keeping you back arched, not letting it bend forward except at the hips. Also with this way you can technically descend almost all the way until your butt gets very close to the ground or ‘ass to the grass’ as they say.
This is basically how “front squats”, “olympic squats” and “overhead squats” are performed. This is also the middle position in Olympic lifts like Cleans and Snatches. These develop the front of the legs, the quads, more than the other kind.
For the first kind, the hip break, you need flexibility in your hamstrings to prevent you from “bowing forward” or bending your spine forward when you come down.
For the second kind, the knee break, you need a lot of flexibility in your ankles and your hips and your butt to descend deep while keeping your upper body vertical. Often when people start these kind, they wear a shoe with a heel or put a plate under their heel and this helps.
That’s my impression on how it all works. There are some good articles on squatting, mostly power or hip break squatting, on this site if you look for them.
knee break - Start off the squat by bending your knees.
hip break - Start off the squat by pushing your ass back.
I believe the knee break is what olympic lifters use and hip break is what power lifters use.[/quote]