Step 1: From a standing position, hold onto the TRX handles with palms facing down, and elbows held up at around 45-75 degrees.
Step 2: Lean back, extending arms, engaging your back muscles and keeping heels on the floor (you are supporting yourself at a 45-degree angle from the standing position)
Step 3: Pull your ab inwards, engaging your core muscles, while tightening your buttock muscles (your body should either form a straight line at a 45-degree angle, with your heels on the floor or a seated position, with knees and hips bent)
Step 4: Pull yourself upwards, focusing on depressing and retracting (ie: bringing together) your shoulder blades, all the while keeping your elbows held out at 45-75 degrees.
Step 5: Lower yourself back to the starting position, in a controlled fashion.
Step 6: Repeat.
How to progress the exercise
In order to have better control of the shoulder stabilizers in this exercise, start performing the exercise with knees and hips bent at 90 degrees (seated position). From there, you can progress to a straight-bodied position, then to a more horizontal position.
Tip: The progression of this exercise will be a lot slower, due to the lack of bigger lat muscle engagement. Due to this, it is better to begin the exercise in a more upright position to better work the smaller shoulder girdle muscles with proper form.