The internal and external cable rotation is an exercise used to strengthen your rotator cuff, in order to prevent shoulder injuries.
The rotator cuff if often injured in the gym. Many exercises put a lot of stress on the shoulder joint, which might easily lead to injury. This exercise is to strengthen the small stabilizing muscles responsible for external rotation.
It is important to emphasis the external rotation over the internal rotation. This is because for most people, the internal rotators are already quite strong from doing bench presses, overhead presses and even everyday movements like typing the computer, while the external rotator remain weak, causing strength imbalances in the rotator cuff.
Don't do this exercise with heavy weights. This exercise works best with light weight and high reps (around 15-20 repetitions). Remember, this is an assistance exercise to strengthen the small stabilizers in your shoulder, not a muscle builder. Using heavy weight will put too much pressure on the shoulder cuff, and it might cause other larger muscles to take over the movement, making this exercise ineffective.
Keep your body, and especially the upper arm still. Make sure to only rotate the shoulder and forearm.
Keep the elbow in place close to your waist. Moving the elbow will take the tension off your external rotator to bigger shoulder muscles.
You can place a rolled towel under your armpit to make it more comfortable on your shoulders.