This curl develops arm size and strength by isolating the bicep. The arm positioning forces the muscle to contract without the assistance of other muscle groups.
Grab a dumbbell with one hand. Sit on the edge of a bench and place your feet wider than shoulder-width. Place the back of your arm with the dumbbell on the inside of your thigh and place the opposite hand on top of the opposite knee for support.
Slowly curl the dumbbell upward as far as possible. Pause at the top and slowly lower it back to the starting position.
Perform the movement with a controlled pace, not allowing momentum to contribute.
Perform the lowering portion of the lift until the arm is fully extended.
Be sure to keep a flat back and lifted chest throughout the movement.