If you are bored of push ups and plank holds, spice up your core training with this exercise. You can do this exercise with a single cable that allows you to control or adjust the height of the cable head; or, use a machine like the one in the photo. I am at iFitFitness in the Merchandise Mart. They have great conventional and functional training equipment.
This is a unique core exercise. It requires you to have a strong plank. If a plank bothers your back, then you are not ready for this exercise. Before beginning, make sure you have aligned the head of the cable machine in a low position. It should line up at about hip level. If you are holding your body in a proper plank, your spine should be straight, your gaze will be in front of your body, and your hips and shoulders will be parallel to the ground. Start on a very low weight. Each time you extend the weight (or cable handle), rotate the palm from the up position to facing down towards the floor. The balancing arm must be directly below your shoulder. To help develop a solid base, spread your fingers apart (as you would in yoga) pressing the pads of the palm completely on the ground. Extend the arm in a straight line. At the full extension, the arm will also be parallel to the ground. If this range bothers your shoulder joint, please adjust so you are working within a comfortable range of motion.
Test this exercise out before adding it to your workout. There is technique and skill to this exercise. It is worth the effort. It takes planks to a new level of training.