If you place a finger on the tendon under your throwing arm's elbow and simulate the hand motion for a 2-seamer (right hand slide off the left side of the baseball), you shouldn't feel any stress on that tendon. Simulating a cutter (right hand slide off the right side of the baseball), you will likely feel significant movement in that under-elbow tendon. The first time I experimented with a cutter, I struck out the side in 9 pitches. I threw it effectively for a few more outings and then it started taking a ridiculous amount of time between starts for my under elbow tendon to bounce back. With a curve ball, you cushion your elbow to protect it (by pointing your pinky in the air when you bring your arm back). With a cutter, you bring your arm back with a fastball grip. It adds to the deception, but you lose the cushioning of your elbow. I kept throwing the cutter even after experiencing the discomfort it was causing because it was so effective. However, I would limit myself to a few cutters per game in key situations. These are my first-hand experiences, and it seems to jive with what I've witnessed from major leaguers. The exception, of course is Mariano Rivera. Perhaps he had a trick to cushion his elbow or maybe he was born with a bionic tendon.