This movement is very similar to a full squat, just done with a shorter range of motion. This movement will build muscles of your upper thigh, which include the quadriceps, hamstrings, and glutes.
This movement should be performed in a power rack for safety reasons. Begin by placing the bar across your back, and allow the bar to sit on your traps. Step out of the rack and the same stance as your regular squat. Begin by pushing your hips back and bend through your knees until you reach half the distance of a parallel squat. Once you reach this position, push through your heels and return to the starting position. Repeat this movement for a set number of repetitions, and then return the bar to the rack.
This exercise should be done with heavier loads than a normal squat. This exercise should be done less often than a parallel squat, because the heavier weights will place more stress on your body.
This exercise can also be done by starting at the bottom position. Set the safety pins of a squat rack, so that they will be half way down your squat range of motion. Then place the bar on the safety pins. Squat under the bar and position the bar under your back. Next, stand up as if it you were squatting the bar. Return the bar to the safety pins and repeat.