For a post about low back pain possibly related to your squat, you sure picked an interesting angle to film, one in which we cannot see your back.
However, never fear, the SS Staff Coaches are still here. You're relaxing your low back and hamstrings and letting yourself plummet into the bottom. Can't say for sure, but this could be the cause of back pain. You need to stay tight, get just below parallel, and then drive your hips up from there. I can't tell you more precisely from here but I'd estimate you need to cut about 6 inches of depth off your squat. It will probably feel like you're squatting high at first. But film yourself, so you can see. Crease of hip just below top of kneecap.
So, do this:
1. Take a stance a couple inches wider and turn your toes out more to accommodate this wider stance
2. Make sure you know how to set your back - read the relevant sections of the book and
watch this video
3. Big breath and set your back before each rep
4. CONTROL YOUR DESCENT, stay tight and drive your hips up when you get just below parallel
5. Repeat for sets of 5
6. Gain about 50lbs