The hidden ball trick in baseball is a deception play where a fielder pretends he does not have the ball, secretly retaining it to tag a baserunner out. Sometimes, in order to pick runners off, infielders will deceive runners. They will pretend that they do not have the baseball -- by hiding it, pretending to throw it, etc. -- then quickly tag the oblivious runner off when he steps off the base. This is a pretty rare feat, and not something that is commonly practiced.