Set-up: Sit on a bench with the heel of one leg elevated onto the bench. Place your elbow on your thigh, flexed to 90 degrees from the upper arm.
Lift: Keeping your elbow fixed on your leg, start the movement with your forearm vertical. Keeping the 90 degree angle at your elbow, lower the dumbbell until your forearm is parallel to the floor. Now raise it back to the start position without the use of momentum.
Emphasis: Do not use momentum. Keep motion in the rest of the body to a minimum. Do not shrug the movement to involve the traps or let the shoulder move forward to engage the deltoids. Any of these deviations will reduce the load on the external rotators.
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.
Training Impact
Muscle Roles in external rotations - sitting, dumbbell
Instructions and effects for the External Rotations - Sitting, Dumbbell exercise 
