With a very wide stance and the toes turned out like you would for a squat, squat with one leg while keeping the other leg straight. Just like in a normal squat, the bending knee should remain approximately aligned with the corresponding foot, and the weight should be balanced evenly over the whole foot. Allow the foot of the straight leg to roll onto the back edge of the heel—with improving mobility, you’ll require less motion of this foot.
 
Stand again so both legs are straight, then squat on the other leg.
 
The Cossack squat can be loaded in numerous ways, most commonly with a barbell on the back or front rack, or kettlebells or dumbbells on one or both shoulders, or one held in front of the chest.
 
Purpose
The Cossack squat improves hip mobility through active and loaded stretching, which means it’s simultaneously developing stability in these greater ranges of motion. It builds unilateral leg and hip strength, especially when loaded, which can further help bilateral leg and hip strength and stability; in particular it strengthens the adductors and inner hamstrings, which are often weak and neglected. It can be used as a training exercises, and as a preparatory exercise in a warm-up to improve mobility and stability in the hips for the session.
 
Programming
As a preparatory exercise, use 2-3 sets of 5-12 reps per side without weight or limited weight. As a training exercise, reps can be dropped to 3-5 to allow heavier loading.
 
Variations
The conventional Cossack squat involves standing up completely between reps. A common variation that improves mobility development is to keep the hips as low as possible as you transition from one leg to the other. Weight can be added in a number of ways, from a barbell on the back or front rack, to a kettlebell or dumbbell held in the goblet position, dumbbells or kettlebells on one or both shoulders, to a barbell or dumbbells or kettlebells held overhead with one or two arms.