Correction, the 90 degree rotation was something I was adding incorrectly, so disregard that part, my bad.
This also explains the plank as one variation, but I’m still confused on the squat-like motion in the UA version vs the lunge version seen everywhere else. The lunge version seems a bit safer from a lifting standpoint because its more controlled than the jerky motion needed to get that foot under you and pop into a squat position.