To build the tricep muscles of the upper arm.
Begin by using a single handle attachment onto the high pulley of a cable machine. Use a grip with your palm facing upward. Begin by pulling the weight down, so that your forearm and upper arm form close to a 90 degree angle, the angle can be less than 90 degrees. This is your starting position. Now, while keeping your elbow close to your body, push the weight down by squeezing your triceps. Continue this movement until your arm is completely straight, hold this position for a second and really squeeze the tricep muscles. Then, slowly return your arm to the starting position.
Make sure to keep your elbow tight to your body to ensure that you are isolating the triceps. Also, the only body part that should be moving is your forearm, the rest of your body should remain stationary.