This is a very hard exercise for many because of the balance that is demanded, but they are an absolute killer for your hamstrings and glutes and deserve a place in your training repertoire. Not only do they target the posterior chain very well, they are also much easier on your lower back than a good morning or a standard Romanian deadlift.
To perform these with the left leg, grab a dumbbell or kettlebell in your right hand and let it hang. You may use a power rack to balance yourself, but don’t grab the rack in an effort to add some help to the exercise. Keep your left leg firmly planted while hinging your hips backward and bending over with the weight in front of you. Use your right leg for counterbalance by letting it rise behind you while keeping it as straight as possible. Switch hands with the dumbbell and do the other leg immediately after you’re finished with the first leg.
The key thing to remember here is that you don’t just fold at the waist, you push your glutes backward while bending over. This is called a hip hinge, and it’s an important move to master since it occurs in deadlifts, kettlebell swings, good mornings, and many of the Olympic lift variations. The basic hinge action is the same in all of them, except here you’re doing it with just one leg.