The side plank with reach under is builds lateral core strength and stability. The added upper body movement increase the demand placed on the shoulders and triceps.
Lie on your side with your bottom elbow on the floor. Raise your hips so that they're off the ground and your body forms a straight line from your ankles to shoulders. Extend your top arm laterally so that it is perpendicular to the floor.
Reach under and past your hips with your top hand, keeping your torso stable. Then reverse motion back to starting position.
Don't allow your hips to drop. Maintain a straight line from ankles to shoulders.
Press your elbow into the ground to create tension in your back and squeeze your glutes as you hold the side plank.
Don't let your shoulders roll forward. Keep your chest up and shoulders back.