Grip one kettlebell firmly in both hands with your arms extended in front of you. Gently lift one foot off the floor a few inches. Brace your core and slowly hinge your hips back, driving the elevated leg behind you as you do so, feeling the stretch in your hamstring on your grounded leg. Pause at when your hips have hinged as much as possible and the kettlebell is almost touching the floor. Slowly return to the start position by driving your hips forward and bringing your leg forward, exhaling as you do so. Repeat all the required reps (or time) on one side before changing to the other.