The hollow rock is a static ab exercise that can be used as some variation for the unweighted plank.
Lie on your back with your arms overhead and crunch your abs to create a “hollow” position—this should elevate your feet and arms from the floor. 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 hollow rock develops strength and stamina for general trunk stability.
The hollow rock should be performed for sets of 10-30+, or can be done for prescribed periods of time, usually 20-60 seconds, and will generally be more of a stamina exercise with higher reps.