The entire point of this exercise is for your body to resist rotation, and to remain in a fixed position (aside from the moving arm). If you can't keep your pelvis and spine from rotating or collapsing, this is a sign of poor lumbo-pelvic stability.
When the barbell is traveling to the left, you should really feel your oblique muscles on the right side of your body working to prevent your body from rotating. Conversely, when the barbell is traveling to the right, you should really feel your oblique muscles on the left side of your body working to prevent your body from rotating.
- Set up a barbell so it is in line with the center of your body, and get into a tall kneeling position. The wider your base is, the easier the exercise will be. Aim to keep your knees approximately hip to shoulder width apart.
- Your head, torso, and hips should be in a stacked position. Your spine should be in neutral alignment (whatever YOUR neutral is), your ribs down, and pelvis level. I like to use the canister analogy.
- Position your arm so it is in a fly position. Your shoulders should be level, and you should have a slight bend in your elbows. Do not allow your shoulders to shrug and elevate towards your ears. Keep them down.
- Before you perform a fly and move the barbell away from the midline of your body, take a deep breath in through your nose (360 degrees of air around your spine), brace your core (pretend that you are about to get punched in the stomach), actively tuck your ribcage towards your hips (lightly close the space in your midsection), and engage your glutes.
- Now perform a fly. Perform the movement through your shoulder. Keep your elbow in a fixed position. Use a range that feels good, and one where you can keep your entire body in a fixed position. If you go too far, you will not be able to resist rotation and will likely lose balance.
- It is extremely important that you never disengage the muscles in your upper body, particularly when the weight is moving away from your midline. This will put your shoulder and elbow in an extremely vulnerable position and will not make them very happy.
- Once you've reached your end range, exhale and return the barbell to the midline of your body. Reset, and perform the fly with your opposite side.
You can make this exercise easier by using less resistance, decreasing your range of motion, or using a standing stance.
You can make this exercise more challenging by using more resistance, increasing your range of motion, adding band resistance (attach on one side to make the anti-rotational component even more challenging), or performing negative reps and taking 3-5 seconds to move the bar away from the midline of your body.