Sit on seat and grasp stirrup to one side. Position elbows out to side, slightly lower than shoulder height. Position hand slightly narrower than elbow width in front of upper arm.
Push stirrups out straight in front of shoulder until arm is straight. Return stirrups to original position until slight stretch is felt through shoulder or chest. Repeat.
Also see exercise performed with twisting movement. Pectoralis Minor and Serratus Anterior act as stabilizers if torso is not against back support as shown.
Stabilizers (see comments)