Step 1: With the TRX straps set at three inches above the ground, place your feet in the stirrups so the tops of your feet are facing the floor.
Step 3: Begin the movement by engaging your shoulder girdle stability with your scapulae depressed.
Step 4: Shift your weight slightly forward so your shoulders are in front of your hands; at the same time, proceed to bring your knees into your chest, focusing on engaging the abdominal’s (not the hip flexors). You are still maintaining a strong plank position with your core and glutes tight.
Step 5: Return back to the starting plank position
Step 6: Repeat the knee to chest movement several times.
How to progress the exercise
Being that one arm is on the floor, there is more stability to this exercise, than doing the Chest Fly. This exercise can be prepared for doing the Chest Fly.
As for any fly exercise, shoulder stability is of great importance and must be the primary focus throughout the movement.