The single-legged Romanian deadlift is one of the best unilateral hip hinge strength exercises. You want to hinge forward from the hip and keep your upper body and the leg that moves backwards as one strong unit from the head to the toe.
However, a lot of people struggle to execute this exercise correctly. They keep their back leg down in order to maintain balance and bend from the lower back rather than performing a hip hinge.
What that does is take the emphasis away from what you're intending to do:
training the hip hinge pattern and the associated muscular activation of the hamstrings and glutes.
So, What's the Solution?
You can use the simple cue of "pulling your toe towards you." By pulling the toe towards you, you immediately place more tension on the back leg and it'll allow you to move the back leg backwards. You'll feel more stable on the ground and lift heavier loads.