The Cossack squat is a leg and hip exercise good for improving both hip stability and mobility.
With a wide stance and the toes turned out somewhat, squat toward one leg while keeping the other leg straight. Just like in a normal squat, the bending knee should remain aligned with the corresponding foot, and weight balanced evenly over the whole foot.
Aim to keep the inner edge of the foot of the straight leg against the floor as mobility improves; initially you will likely need to allow the toes to lift so only the heel remains in contact.
Stand again so both legs are straight before repeating the squat on the other leg.
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. 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.
As a preparatory exercise, use 3-4 sets of 6-12 reps per side without weight or limited weight. As a training exercise, reps can be dropped to 3-5 to allow heavier loading.
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, to a kettlebell or dumbbell held in the goblet position, to a barbell or dumbbells or kettlebells held overhead with one or two arms.