He's stretching his psoas and hip adductor, if these are tighter than the muscles they mirror (such as the gluteus medialus) then the lower back and pelvis will be misaligned.

Muscles don't just move you, they hold you in place and must be balanced.

Think about a door, if the hinges are misaligned then it wilo creak and crack and eventually wear out. Slam the door and you'll do some damage!

Line the hinges up and it works properly.

That's how I would explain balance, stability and mobility in the body.