This is a core exercise that great for athletes. Set your arms in a modified push up position with your knees off the ground. Then, while tightening your core, squeeze your abs and bring one hand off the ground and bring it to your chest.
To get the most out of this exercise, you’ll want to try your hardest to not move your hips. The more stable your hips, the more you will work your core instead of shifting your body weight into your legs. Also, you’ll want to keep your elbows as straight as you can. In this video, I bend my elbows during the box plank. This is not ideal.