When is it in Play?
It is important to recognize when a dropped third strike has occurred. When the catcher catches and holds the ball in his mitt on a third strike, the batter is out. If the ball hits the ground before it hits the catcher's mitt on the third strike (after it is called by the umpire), or the ball passes by the catcher after a wild pitch (at which the batter still swung), the dropped third strike rule is in play.
When the Batter Can Run
As long as there is no one on first base at the time of the dropped third strike, the batter can run to first if there are less than two outs. When there are two outs before the third strike, the batter can run, no matter what.
Other Base Runners
The other base runners must be aware of a dropped third strike, because they are allowed to advance as well. If the batter makes a run to first, the runner occupying first base must get out of the way and advance to second or else one of the runners will be tagged out to officially end the inning. The same applies to other base runners who must move forward as well to avoid an out.
Tag Out
In order to complete the out when a dropped third strike is in play, a member of the opposing team must tag the runner out. This is why some catchers will touch the batter immediately after a third strike to assure that he will not try to make it to first on a fumbled ball. If the ball gets away from the catcher, he must recover it immediately and throw to the first baseman so he can tag the runner out. If the runner makes it safely to first base, the inning continues.