this is from dave tate’s website - www.elitefitnesssystems.com/
Box Squat: The benefits of this exercise are numerous. They develop eccentric and concentric power by breaking the eccentric concentric chain. Box squats are a form of overload and isolation. The box squat is the best way to teach proper form on the squat because it is easy to sit way back while pushing your knees out. To take the bar out of the rack, the hands must be evenly placed on the bar. Secure the bar on the back where it feel the most comfortable. To lift the bar out of the rack, one must push evenly with the legs, arch the back, push your ABS out against the belt, and lift the chest up while driving the head back. A high chest will ensure the bar rests as far back as possible. Slide one foot back then the other, to assume a position to squat. Set your feet up in a wide stance position. Point your toes straight ahead or slightly outward. Also keep your elbows pulled under the bar. When one is ready for the decent, make sure to keep the same arched back position. Pull your shoulders together and push your ABS out. To begin the decent push your hips back first. As you sit back push your knees out to the sides to ensure maximum hip involvement. Once one reaches the box, you need to sit on the box and release the hip flexors. Keep the back arched and abs pushed out while driving your knees out to the side… To begin the ascent, push out on the belt, arch the back as much as possible, and drive the head, chest, and shoulders to the rear. If you push with the leg first, your buttocks will raise first, forcing the bar over the knees, as in a good morning, causing stress to the lower back and knees and diminishing the power of the squat.