Hold resistance band, doubled up or single, with both hands in underhand grip at about shoulder width apart (vary hand position to adjust load).
With the arms locked at the elbows, pull the band apart by driving the hands away from each other until the band contacts the chest, hold at end range for 2-3s, then release tension back to start position. All movement should be in a controlled manner, and the focus should be on squeezing the shoulder blades together, and activating the posterior muscles of the shoulder complex.