Doesn't this movement look familiar, perhaps similar to the side bridge? Unfortunately some people cannot perform a side bridge on the ground because it hurts their shoulders too much. No problem! Here's your alternative.
Side bridges are LEGIT because it's an exercise that demands stability of the pelvis, hips, and the rib cage. What a lot of people don't understand is which muscles are working. In this video I help point it out; all the muscles that help stabilize the pelvis and rib cage on the LEFT side of my body are working here. The obliques, QL, glute med, TFL are heavy hitters here. ALSO my right adductors are working too, it's all about pelvic control here. As my body bends away from the TRX anchor, these muscles are working in an eccentric fashion (I always like to think of a crane lowering a heavy cargo box). As I push my pelvis back towards the TRX anchor, these muscles change action and begin to work concentrically (lifting the cargo box into the air).