Re: Yet another "starting strength" and "how's my back squat" thread.
In the sets of 275 you are starting the movement with your hips and knees at the same time. This is whats getting you forward.
You need to send the hips back at the very start of the squat. Think about how you would close a car door using your butt.
Hips first, knees second.
(i only watched that set)