Begin in a power rack with a box at the appropriate height behind you. Typically, you would aim for a box height that brings you to a parallel squat, but you can train higher or lower if desired.
To set up the chains, begin by looping the leader chain over the sleeves of the bar. The heavy chain should be attached using a snap hook. Adjust the length of the lead chain so that a few links are still on the floor at the top of the movement.
Begin by stepping under the bar and placing it across the back of the shoulders. Squeeze your shoulder blades together and rotate your elbows forward, attempting to bend the bar across your shoulders. Remove the bar from the rack, creating a tight arch in your lower back, and step back into position. Place your feet wider for more emphasis on the back, glutes, adductors, and hamstrings, or closer together for more quad development. Keep your head facing forward.
With your back, shoulders, and core tight, push your knees and butt out and you begin your descent. Sit back with your hips until you are seated on the box. Ideally, your shins should be perpendicular to the ground. Pause when you reach the box, and relax the hip flexors. Never bounce off of a box.
Keeping the weight on your heels and pushing your feet and knees out, drive upward off of the box as you lead the movement with your head. Continue upward, maintaining tightness head to toe.