In your scenario, if the third basemen steps on third, the runner from second is out, but the runner from first has to go to second, because otherwise if he stays at first and the batter gets to first, if the ball is thrown to first the runner originally on first will be out. The "ahead of the force" thing applies to bases ahead of the base tagged for the force. For example, bases are loaded with no outs, Graffy playing 2B gets a ball and steps on second to force the runner from first, then throws home. The runner on second can now continue running to third or go back to second, because second base is unoccupied. The runner from third can now go back to third or continue going home, because the runner from second no longer has to go to third if he doesn't want to. If they both end up on third, hopefully we don't throw the ball into left field and someone is tagged out, but the runners are no longer forced to go to a specific base. With the Cirillo play, first base was tagged, so Berkman did not have to go to second, Biggio didn't have to go to third, and the guy on third didn't have to go home. That guy and Biggio decided to go anyway, but Berkman went back to first and was thus safe because first was unoccupied by the batter, who was out.