This is an excerpt from Powerlifting-2nd Edition by Dan Austin & James Bryan Mann.
Powerlifting focus: Keep the chest up and knees in line with the ankles during the squat.
Muscle target: Quadriceps, gluteus medius, gluteus maximus, tensor fasciae latae, and hamstrings.
Starting position: Stand in an erect position, holding the bar in either a clean grip or with the arms crossed. Feet are shoulder-width apart, and the toes are either pointed straight ahead or slightly turned out (figure 8.2a).
Execution: Push the hips back slightly and squat down (figure 8.2b). The chest should stay upright throughout the entire descent. Once the crease of the hips goes below the crest of the knee, return to the starting position.
Coaching points: Keep the elbows high during this exercise. If not, the bar may slip out of position. The purpose of this exercise is to keep the chest tall and break or prevent the habit of leaning forward. It may also be beneficial to shrug the shoulders during the exercise; this creates more of a shelf for the bar to sit in. Again, pay special attention to the heels. At this point, they might be able to remain flat on the ground.