Open the groin.

Push your hips back just enough to open your groin, without moving the bar.
Hips back + groin open
KEEP THE WATER IN THE BUCKET
KEEP THE WATER IN THE BUCKET
BOTH ROPES PULL AT THE SAME TIME
BOTH ROPES PULL AT THE SAME TIME
BREAK KNEES AND HIPS TOGETHER
BREAK KNEES AND HIPS TOGETHER
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.
Super Training
First thing I do is sit my hips back... It's important to note that the sitting of the hips back does not move the bar. The bar stays in place while the hips break.
John Paul Cauchi
A short movement of the hips backwards and then squat down from there. “Back, back, back” isn’t for you, it is more “down, down, down”.
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.
To initiate the squat, think about sitting back and pushing your knees out to the side. This will help you load the glutes and hamstrings, as well as helping keeping your lower back healthy.
Robertson Training Systems
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.
Alan Thrall
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.
Renaissance Periodization
To initiate the descent, I think about basically two things: I drive my hips back and I drive my knees out... You do not want to lead with your knees.
Super Training