HOW: While standing, place the box behind you. Lift one leg straight in front of you, and then sit back with good squat form, trying to be as controlled as you can. Try to tap the box with your butt without completely sitting down, and then come back up using the same leg.
FEEL: You should feel all the muscles in the loaded leg working.
COMPENSATION: Avoid letting the knee cave in, and try your best to maintain a neutral spine throughout the movement.