Step 1: From a side-lying position, with your forearm on the floor, place one foot in each of the TRX stirrups; one foot should be behind the other. Your head should be aligned in a straight line with your body (ie: centered in between both shoulders). Your shoulders should be stacked above each other, and stacked above your elbow. The other arm is extended upward towards the ceiling; also stacked above the shoulders and elbow of the grounded arm. Keep your core fully engaged to avoid the hips drooping.
Step 2: From this position, rotate your torso so you are able to bring your upper arm down and through the space underneath your torso from front to back, then rotate back up to the starting position.
Step 3: Repeat this movement several times, then switch sides and repeat.
How to progress the exercise
Progressing this exercise to your hands as opposed to your forearms will increase the challenge for your shoulders while continuing to work your core. Make sure to shorten the length of the TRX straps to assure you remain parallel to the floor. Incorporating a BOSU or Stability ball under your forearms or hands will bump up the challenge!