Lie on your back with your arms overhead and crunch your abs to create a “hollow” position—this should push your lower back flat into the floor and elevate your feet and upper back. Shift your weight to begin rocking back and forth slightly while maintaining the hollow position—don’t allow yourself to straighten from this crunched position. The better you can maintain the hollow position, the smoother the rocking will be.
 
Purpose
The hollow rock is an isometric trunk exercise that develops stamina and control for general trunk stability.  
 
Programming
The hollow rock should be performed for high reps, or can be done for prescribed periods of time, usually 20-60 seconds.