In baseball, an unassisted triple play occurs when a defensive player makes all three putouts by himself in one continuous play, without any teammates touching the ball (assist). In Major League Baseball, it is the rarest of individual feats, along with hitting four home runs in one game. There have been more perfect games in baseball history than unassisted triple plays. During a span of over 65 seasons (June 1927 to September 1992), there was only one unassisted triple play in the major leagues, made in July 1968. Even “ordinary” (assisted) triple plays are fairly rare, occurring a few times per year.

Specific conditions must be met for the play even to be possible. There must be no outs in the inning and at least two runners on base (usually only two) who are running with the pitch, as in a hit and run. Finally, the batter must make excellent contact with the pitch, resulting in a line drive hit directly at an infielder, usually either the shortstop or second baseman.

http://en.wikipedia.org/wiki/Unassisted_triple_play

Great video explaining how a differential gear works. Now I know!