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.
If you've been around weight training for any length of time, you've probably read or tried an arm specialization routine. There are a bunch of these around with different cool names for what amounts to the same old shit: do a bunch of arm exercises. Yeah, the exercises change and the sets and reps change, but it still amounts to just doing more arm work.