Raised Heel Squat
Performing squats with your heels raised 1 - 2 inches on a solid platform will force your quadriceps to work through a different range of motion, as your knees are extended past your toes. Unless your training for some sports specific reason, I would advise not to perform this exercise, however if you do, take care to not let your knees extend to far past your toes.
For good balance you should aim to perform this barbell quadriceps exercise with your legs wide, making sure the balls of your feet are in contact with the floor throughout, having the barbell placed behind your neck, also in a wide arm grip.
Focus on keeping your back straight, and lower your buttocks down vertically to the ground, allowing your knees to naturally take the direction they want to travel over your toes.
Beginners should use a lightweight, those using heavier weights should take care to warm-up the muscles prior to extending the knees out past your toes.
Contract your buttocks, and push down with your heels to smoothly lift yourself back up to the starting position, and repeat for desired amount of reps.