Personally, the video you are about to watch (brought to my attention by figgifig) is pretty amazing: I would say this is pretty conclusive evidence of why Yankees pitcher Mariano Rivera is able to throw only one pitch that has unpredictable, yet precise movement.

He has mastered the Angels so far in the ALCS but has Major League Baseball gotten a look at how he "warms up" for his appearance on the mound? You be the judge...

With a tie game in the bottom of the 10th, the Yankees bring in Rivera.

After warming up, he has his back to home plate so the home plate ump can't see.

He looks down at the ball, then up to see if the 2nd base ump is watching.

He then looks back at the ball and appears to spit on it.

And when he tests clean for steroids they say he is not a cheater.

The spitball was banned by baseball in the early 1920s. It gave the pitcher an unfair advantage with the "funny" movement that the pitch seemed to make, and was deemed unsafe to batters as the ball was not as visible when marred with foreign substances.

and a link to DETAILED SCREENGRABS of this infraction.