Support your torso on a bench with one hand and knee while holding a dumbbell in the other hand. Keeping a slight bend in the elbow, raise the dumbbell outward and back in a reverse fly movement, focusing on engaging the rear shoulder and upper back, then return slowly.
Log your dumbbell one arm reverse fly (with support) (version 2) sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.