This exercise is a great alternative to the Preacher Curl.
Stand in front of an incline bench so your arm is resting on the top of the angled portion of the bench.
Grasp a dumbbell in your hand with palms facing up.
Rest your arm against the back 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.
Note: Perform this exercise in a slow controlled manner for best results.