A Preacher Curl uses a special bench to support the triceps and isolate the biceps to build the arms. This version uses dumbbells and single arm movements for better control.
Adjust the seat of the bench so your arm is level with the top of the bench.
Grasp a dumbbell in your hand with an underhand (palms facing up) rest your arm against the bench and extend it fully down.
Keeping your arm on the bench at all times, curl the dumbbell up towards your head.
Pause for a moment and then lower the dumbbell back to starting position.
Switch arms and repeat.
Perform this exercise in a slow controlled manner for best results.