This exercise is used to replicate the squat and get help to build mass in your upper thighs. This movement will allow you to use heavier weights than squats, because you do not have to rest the weight across your back.
Sit down on a leg press machine and place your feet on the platform above you. You should use a shoulder width stance and place your feet at the top of the platform. This is so your toes do not go over your knees. Next, release the safety handles so that weight is free to move. You should then press the weight up but do not lock out your knees. Now that you are in the starting position, you can begin the exercise. Begin by lowering the weight and bringing your knees to your chest. You should lower the weight until your upper thigh forms a 90 degree angle with your lower leg; this is the same as going to parallel in a squat. Now, push through the middle of your foot and raise the weight back to the starting position. Repeat this movement until you completed the amount of reps in your set. Make sure to lock the safety pins into place, so the weight does not fall on you.
This movement should not replace squats in your workout, but can be done to make you a better squatter.
You can move your feet around to where you feel most comfortable. You can also point your toes slightly outward if it feels more comfortable.