HOW: Set-up a box or object behind you, the taller it is the easier the exercise will be. Lift one leg straight in front of you, and then sit back with good squat form only on one leg, trying to be as controlled as you can. Sit down completely on the box, and then stand back up with the same leg. Repeat
FEEL: You should feel all the muscles in the leg on the ground working.
COMPENSATION: Avoid letting the knee cave in, and try your best to maintain a neutral spine throughout the movement, do not lean too much side to side, forward, or backward. Avoid using too much momentum to stand up or 'plopping' your weight onto the box