Can be a common mistake for golfers to slide their hips back and/or not turn them enough on the backswing. The rear hip has to keep "cranking" back, helps sequence the arms with the torso turn and sets up a lot of good pieces to achieve Key#2 (weight forward at impact).
To do this the lead knee has to increase in flex and rotate inwards a bit as the rear leg decreases in flex. Note that due to this "knee linkage" each player's hips are level at A1 (address) and their rear hip is higher than their lead hip at A4 (top of the backswing). You'll also notice that each player has their rear hip pocket reach the center line.
If you wanted to check these alignments for yourself but it's a pain in the butt (pun intended) to film a posterior view, you can check it like this.
At A1 the rear hip joint is over the rear foot and at A4 it will be inside the rear heel (half a ball to a ball).