Stand or kneel with one hand and forearm braced on a bench or stable surface, holding a dumbbell in the opposite hand. With a slight bend in the elbow, lift the dumbbell outward and slightly upward in a controlled arc until your arm is roughly parallel to the floor, focusing on squeezing the rear shoulder and upper back. Slowly lower back to the starting position with control, keeping your torso stable and avoiding momentum.
Log your dumbbell one arm reverse fly with support sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.