I know I should (and eventually will) put up a video, but bear with me for now.
- Start in a plank position (facedown, forearms flat on the floor, body rigid, like a pushup position only your whole forearm - from hand to elbow - 0touching the ground).
- Lift your right arm and place your hand on the ground underneath your shoulder.
- Push up, letting your left arm straighten so you end up in a pushup top position.
- Supporting yourself with your left arm, pick up your right hand.
- Lower yourself onto your right forearm.
- Lower your left arm so you return to the start position.
- Repeat, switching left and right.
I love this exercise. First, it's a nice arm exercise - as I wrote above, if you do it right you're doing the top half of a one arm pushup with every rep. Keep your shoulders and neck packed and you're doing a lot for coordinating your posture, and stabilizing the shoulder.
But the real benefit of this movement is the kind of core training you're getting. This is like a plank - you are training your core to hold your body stiff, but you're not just stopping your body from collapsing in the middle (resisting spinal extension) but also stopping your body from twisting (also known as anti-rotation). As soon as you pick up one of your hands - step 2 - your right shoulder wants to twist towards the ground. The only thing keeping your body straight is your core!
The other beautiful thing is that your weight is constantly shifting from one arm to the other. It's much more dynamic than, for example, just holding a pushup position on one hand. Every time you shift your weight from one supporting hand to the other your core musculature has to respond, dynamically. Which is exactly what you want your core to be good at doing - quickly adjusting to a changing load of forces. After all, when you throw a punch, you don't push with that arm overseveral minutes - you shift from no rotational forces to heavy rotational forces back to none in a very short time. If you want your core to support punching it has to be good at resisting rotation quickly, and then turning back off when the punch is finished.
Your weight shifts back and forth (left to right) multiple times with each rep. All in all, a very taxing workout for your core!
If you need to, feel free to wear a weight vest while you do it or put your feet up on a Swiss ball or something to add some challenge. You could also keep your hands/ forearms on a Swiss ball but I don't think you could use a TRX for it.
Just please don't do 100 reps with each arm! As always, when strength training, harder is better than longer.