This functional gymnastics movement is great for reinforcing good anterior core position. There are two variations, the rock (with movement) and the hold. Both are very challenging. Ten to twenty seconds will be tough for most athletes.
If you catch yourself compensating by bringing the arms up, scale it back a bit by doing the movement with the knees bent. As you get stronger, begin to extend the legs.
Make sure your arms are fully extended and the toes are pointed. Note that the heels do not touch the floor in the hollow rock variation.