I've never understood why bodybuilders apply 21's only to their biceps. The way I see it, 21's are a simple and versatile overload technique that can easily be applied to almost any strength exercise.
As a refresher, 21's are usually performed by doing 7 partial reps at the top of the range of motion, 7 more partial reps at the bottom range, and finally 7 full-range reps.
But, as with all other overload methods, certain techniques work better with some movements more than others, like focusing on the mid-range of the movement.
You can do this either seated or standing. Since the lever arm is the longest in this exercise when the humerus is parallel to the floor, the isometric is performed in that position.