Purpose of Exercise: To develop the quadriceps. When you do squats on a machine, you can work the thighs intensely while putting less strain on other areas such as the knees and lower back. There are a number of machines designed to do this.
Place your shoulders under the bar, come to a standing position. Position your feet in the desired effect you want to work the thighs. Bend your knees and squat down until your thighs are lower than parallel. Now press back up keeping constant tension on the thighs until you return to starting position.
Insider's Tip: Standing with your feet moved forward helps isolate the quadriceps, especially the lower area below the knee. It also takes pressure off of the lower back because you will not have to bend forward during the movement.