Sit on a bench with a single arm outstretched in front of you just below the level of your shoulder gripping the handle at shoulder-width with an inwards facing palm.
Exhale and bring your arm back by driving your elbow behind you and keeping it tucked in.
Continue to pull your arm back, simultaneously driving your shoulder blade back as well until your elbow passes your torso.
Inhale and allow your arm to extend, returning to the starting position.
After all reps are completed, switch sides and repeat the movement. 10 reps prescribed means you should perform 10 reps on each side (20 total).