Typically done with runners at 1st and 3rd.  When the pitcher comes set, the runner starts in motion towards 2nd.  The goal is one of two outcomes - 1) the pitcher steps off and throws over to 1B, the runner continues to 2nd but if the 1B throws to 2nd the runner stops and gets in a pickle so the runner from 3rd can score (if no throw to 2nd then the runner just continues to 2nd).         2) the pitcher is caught off guard by the early steal and balks and everyone advances 1 base.