With a runner on 1B, the runner will steal on the pitch.  The batter will swing at the pitch no matter where it is, in order to put the ball in play.  If executed correctly the batter will hit the ball behind the runner.  For example, the runner is stealing from 1B the batter hits the ball into the gap that is left by the 2B because they are going over to cover 2nd.  The perfect situation would then allow the runner to continue on to 3B.