Sitting Rotator:

This apparatus has three rotational seats which swivel to stretch the external and internal obliques and the abdominals. It serves, also, to align the spine and lower back.

Grasp the central bar with both hands in order to keep the shoulders square. Place your feet on, or under the foot rests. Twist your hips from side to side, being sure to keep the shoulders stationary throughout the motion. Slowly extend the range of movement to each side, sometimes holding at the maximum stretch.

Installation Drawing (pdf)