Open up the groin... Open up your taint... [When you] push your knees out, you feel... a lot of strain on your knee. Instead, open up way up high in your groin. When you concentrate and open it up back here it makes you sit back and... you don't feel any stress on your knees... You end up using more hip and hamstring.
If I try to push my knees out too far, I don't sit back but I feel it on my knees... if I when I break my hips it'll naturally start to open up your groin way up high. So if that stays open, your knees naturally come out so there's less strain on them.
Regular trainees will often exaggerate [the 'sit back'] cue, sitting back at the hips and maintaining a near vertical shin angle while they descend... A very useful clue that something is wrong during the descent is the distribution of weight across the feet. Ideally, the combined center of mass of the barbell/lifter system is held in perfect balance over the mid foot throughout the lift.
Pushing your hips back while unlocking your knees instead of just unlocking your knees and going down is a good cue to use here. So when you think of squatting think of going a little bit back and then plenty down and think of actively staying on your whole foot heels and toes.