The Hip-Hinge isolates hip flexion & extension for glute & high hamstring development. It also works the entire backside of your body, or posterior chain. Though this exercise can be done with the legs straight, it’s typically performed with soft knees. Think about closing a car door with your butt. I recommend using the double-legged variation to build size & strength and the single-legged variation for balance, mobility & stability.
To perform the double-legged variation, hold a pair of dumbbells and slide them down your legs until they reach knee or upper shin level (or when your trunk is parallel to the floor). Only go lower if you can maintain a slight natural arch in your lower back. Go slowly and never force the range of motion. You can also do this with a kettlebell between your legs.
To perform the single-legged variation, start by using both hands for self-assistance so you can keep your hips & shoulders perfectly square to the ground. Add a knee lift to strengthen the hip flexors. Progress to only using single-arm self-assistance. Progress further by holding a weight in the non-assisting hand. If you want to put weight in both hands, kick-stand the non-working leg on the floor, a box or onto a wall.
For best results, add a mini-band to both drills. You can also perform them with your feet on slant boards to increase ankle & hip mobility.