In an incredibly insightful video by The New York Times, motion and spin analysts explain Rivera's secrets.
First of all, Rivera uses mostly one pitch, a cut fastball, which he occasionally spells with a traditional fastball.
His arm motion and the spin produced on the ball are identical in both pitches, so the batter has no clue whether he will be subjected to the cutter's sharp lateral motion or not. Many other cutter pitchers throw the pitch like a slider, which shortens their careers by straining their arm and spins in a different different direction than a fastball, allowing batters to know what is coming.
Because of Rivera's unique cutter, he has been able to be incredibly dominant for a decade and a half.
Perhaps what is most amazing is that nobody has been able to replicate this trump card of his.
