A Preacher Curl uses a special bench to support the triceps and isolate the biceps to build the arms. This version of the exercise uses dumbbells.
Adjust the seat of the bench so your arms are level with the top of the bench. Grasp a dumbbell in each hand with an underhand (palms facing up) grip. Picking up the dumbbells, rest your arms against the bench and extend them fully towards the floor.
Keeping your arms on the bench at all times, curl the weight up towards your head. Pause for a moment and then lower the weights back to starting position.
Perform this exercise in a slow controlled manner for best results