This exercise is a great way to make a traditional Plank way more challenging. You have to hold the plank position with your feet elevated in the rings, and move the stack of plates from one side to the other. There’s a great anti-extension and anti-rotation training component here, which are two critical skills needed by athletes to keep a strong and stable core. Do this one only if you can move the plates without your feet suspended in the rings.

http://www.stack.com/a/2-advanced-core-exercises