If your back hurts, it's because your knees are going too far forward compared to the center of your feet when you go down, putting strain on your back. When going down, push your hips back and as your hands pass over your knees, try to keep your shins as vertical as possible and extend your arms forward while thinking about going down as much as you can without your knees going forward. You must never let your knees go past the center of your feet, and remember to push your hips back when going down! In the lowered position, extend your arms forward as much as possible; just remember this! If your back hurts when you go up, it's because your center is leaning back. When going up, your center should slightly move forward.