HOW: In a standing position, shift your weight to one leg. Apply pressure through all of your foot and slightly bend that knee. Hinge forward at the waist while keeping your back flat and kick the opposite leg back. Pull yourself back to the starting position by pushing into the ground and using your glute and hip muscles to bring your back up straight again.
FEEL: You should feel your hamstring muscles working as you hinge forwards and back up. You should also feel your foot/ankle muscles working to balance your leg while hinging forward.
COMPENSATION: Keep your back flat as you hinge forward at the waist, don't twist it. Keep your hips level, don't twist them as you kick your leg back.