Planks train your core in ways few other exercises can do. Planks with rotations increase the magnitude of the challenge and bring additional muscle groups into play. 

What's involved: Core, triceps, shoulders, quads, lower back, lower abs, upper abs, lateral abs. 

Powered by: Core, lateral abs, lower abs.

Tip: While performing planks with rotations, consciously tighten your lower stomach flattening the muscles there and forcing them to align better.