On the 7th day of Rulon Rules, Coach Jen gave to me:
Learn How to do a Hollow Rock
Hollow Rocks will strength the rectus abdominis and some of your obliques. As triathletes, we can have weak core, so doing hollow rocks will help stability, flexibility, and injury prevention. It is considered an advanced move, so check out this great video by Carl Paloi who shows hollow rocks, but will also show a progression if you are not able to do the whole exercise.