@actionhiro: Typically ankle mobility is the issue but it can also be tight glutes and weak hip flexors when you’re trying to sit in a deep squat. This drill works on active end range hip flexion with an externally rotated leg: when you draw your leg towards your chest you’re engaging your hip flexor and actively stretching your glutes. Kicking your leg out and folding helps stretch your hamstrings, which play a minor role in contributing to potential tightness at the bottom of a squat