Back Extension:

The Back Extension equipment strengthens the lower erector spinae muscles of the lower back, as well as the gluteus and hamstring areas. This will also have the effect of improving posture and stability.
To perform the exercise, lie forward on the back extension bench with your thighs resting against the green pad and the heels held in the foot bracket. With your hands behind your head, or arms across the chest, slowly raise your upper body to a high comfortable position, extending the lower back until the lumbar curve is increased. Hold briefly, then slowly lower to the starting position. Perform this carefully to protect the lower back.

Installation Drawing (pdf)