If the pelvis is square to the ball-target line at address, and then 45 degrees angled to the ball-target line at the end of the backswing, then it has rotated 45 degrees. How it got to that 45 degree rotated position could vary from a biomechanical mechanism perspective. For example, in my diagram I show how the right femoral head has moved backwards and leftwards. That could occur via different biomechanical mechanisms. One could use pelvic girdle muscles to pull the right pelvis back and secondarily the right femoral head back (because of its snug-relationship to the hip socket joint). Alternatively, one could shift the torso right-laterally against the resistance of a "fixed" right knee => that causes the deflected forces to slant the right thigh to the left => that drives the right femoral head left-backwards => then the pelvis moves secondarily to the movement of the right femoral head.
I only tried the drill once to understand the biomechanics. I think that BM was using that drill to teach golfers not to sway the pelvis laterally, but to rotate the pelvis.
Brady Riggs is teaching the same basic principle is this swing video lesson.
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