The 'Hip Hinge' is not an exercise to increase strength or build flexibility, it is an exercise to activate our hips to flex. When bending over to lift up a box, change a baby or brush your teeth, a common error to make is to bend your lower back instead of your hips.
This exercise is demonstrated in the clip below.
https://www.facebook.com/CityCentreChiro/videos/1697772190259526/