You know those muscles that pop out first when you start to develop some abs? The ones along the sides? Those are your obliques, and they will benefit from the side plank crunch.
Start in a traditional straight-arm plank position. Drop your right knee and shift your weight to your right side. Extend your left leg straight over your right and extend your left arm straight up in the air. Create a diagonal with your body from left toe to your head, with hips squared off to the left.
Slowly bring your left knee and left elbow together in front of you, balancing on your right hand and right knee. Hold for a second, then re-extend. Repeat for 30 seconds on your left, then return to your original straight-arm plank before switching to the other side.
Focus mentally on your obliques during the side plank crunch, and soon you’ll tighten up your sidelines.