Dumbbell seated reverse grip one arm overhead tricep extension

Sit on a bench with your back straight and feet flat on the ground. Hold a dumbbell with an underhand grip and extend your arm straight up overhead. Lower the dumbbell behind your head by bending your elbow, keeping your upper arm stationary. Pause for a moment, then extend your arm back up to the starting position. Repeat for the desired number of repetitions, then switch arms.