>>76642785
>>76642787
The glute medius wraps around your hips essentially and is only really activated when you abduct or turn. It's higher up on the ass and because yours is underdeveloped there's a divot from your lower back to your ass.
As far as exercises you can do to fix that. Heavy band walks after your squats and cable wood chops.
This muscle is why MLB players and softball players all have big asses because they rotate to swing the bat and throw.