A discussion of finishing the throwing action begins by recognizing that the shoulder needs to drive  through release and after release. If you look at the following picture of Paul you will see that his shoulder continued after release to a position where his back is nearly horizontal. By continuing to drive the shoulder through release you can make minor adjustments in your release point that are subtle movements of the fingers.
After the arm has been accelerated to maximum velocity by the lower body and upper body, the finish of the throwing action (after release of the baseball) needs to reduce as much stress as possible from the arm and shoulder. In order to accomplish this stress reduction, the player needs to take as long (in time) as possible to reduce the speed of the arm from maximum to zero. The path the arm takes during the deceleration process, therefore, needs to be as long (in distance) as possible. The throwing arm moves outside the knee of the lead leg as the arm slows down. At this point, the majority of the stress is distributed across the shoulder blades and down through the back.

An abrupt finish to a maximum effort throwing motion can cause discomfort related to biceps tendonitis. Biceps tendonitis is characterized by a dull pain at the outside top of the shoulder (the attachment point of the biceps tendon). Sometimes the discomfort can be localized in the meat of the muscle of the biceps itself. A lot of sidearm throwers suffer from biceps tendonitis because they finish abruptly coming across their body (remember, the throwing slot is in line with the shoulders so if the shoulders are horizontal at release the finish will also be in line with the shoulders).
Even three-quarters and overhand throwers can develop biceps tendonitis if they throw against a front leg that is very stiff at the release point. The stiff front leg causes a shorter deceleration time for the arm. An ideal finish position for the end of the throwing action has the front leg with a slight bend and the upper body bent at the waist to allow the arm more distance and time to decelerate. The thrower bends at the waist after release of the ball and the back leg comes even with or past the front leg as the motion comes to completion. Both of these actions spread the deceleration force across the back rather than focusing it in the outside top of the throwing arm.
Many pitchers reduce the amount of spinoff at the end of the pitching motion by stiffening the landing leg. This movement is okay if the knee is bent through the deceleration phase of the arm. My concern is always that the knee can straighten too early and cause the previously described abrupt deceleration of the arm.

The throwing motion should finish as long in time and distance as possible…