In core training we often perform the upper body crunch movement and neglect the important lower body extension that strengthens lower abdominal muscles. (blog.bridgeathletic.com)
Each name brand will adjust slightly differently, but climb on first to see if the height of the upper body extensions is comfortable for your arm span. (topexpertchoices.com)
To perform Hollow Body Extensions, lay on your back and lift your hands and feet upwards towards each other with your lower back pressed strongly into the ground. (onnit.com)