This exercise is a staple in any strength training program. If lower back issues are present, modifications should be made to decrease chance of injury and increase probability of strength improvements.
Starting position: place the bar on the back across the scapluas (low bar placement). Engage the core and hinge the hips. Lower the torso towards the ground while maintaining the shins in a vertical position. Squeeze the glutes and hamstrings to raise the torso back to the starting position.
Due to its bar placement, good mornings are considered a core exercise. The lower back is an areas of the body that gains strength through movement of other muscles. This exercise is to build stability in the lower back during spinal loading (bar on the back) and increase glute/hamstring strength.
For strength: complete 4-8 repetitions at 50% of your maximum back squat.