This is one of the most underestimated core exercises in existence. It'll strengthen the obliques, erectors, multifidi, and even your glutes. You can use a rope, a chain, or a long bar to perform this movement. Toy around with different body positions and vectors. All work very well. Make sure you repeat with the opposite side.

Related:  3 Cable Exercises for Strong Abs & Obliques

Related:  The Best Core Exercise. Period.