Use a full range of motion and do not lock your elbow out at the top of the movement.
Keep your upper arms as still as possible, allowing your forearms to drive the movement.
Instructions
Get into the starting position for the standing two arm dumbbell extension by securing a dumbbell with both hands with your palms facing up and holding it above your head.
Your elbows and knees should be slightly bent and your feet should be around shoulder width apart. This is the starting position.
Begin the movement by bending at the elbows only and slowly lowering the dumbbell behind your neck.
Once the dumbbell is down as far as comfortably possible, slowly raise it back to the starting position.
Do not lock your elbows out at the top of the movement, and then repeat for desired reps.