What is the rule about running out of the base path?

An Infield Fly is a fair fly ball (not a line drive or bunt) that, in the judgment of the umpire, can be caught by an infielder, pitcher, or catcher with ordinary effort and when there are runners on first and second or first, second, and third and less than two outs. When the umpire calls "infield fly” the batter is out, regardless of whether the ball is subsequently caught or dropped. The ball is live, and runners already on base may advance (at their own risk) if the ball is not caught or tag up and advance if it is caught. Simply put - it's in effect if there is a force at 3rd base with less than two outs. Why does it exist? To prevent double or triple plays on popups. Imagine bases loaded and a soft popup to third base. Without the infield fly rule, the runners have a dilemma. If they stay close to their bases, the third base player could choose to let the ball drop and get multiple outs by tagging the runner who was on third, stepping on third base to force out the runner from second, and throwing to second base to force out the runner from first. If the runners go too far from their bases, the third base player catches the ball and doubles them off. The reason the rule applies only with at least first and second base occupied is that if there are not multiple runners on base, a missed popup can only result in one force out.

This rule applies when first base is open, or if there are two outs. If the batter strikes out (swinging or looking) and the catcher does not catch the pitch before it hits the ground, the batter can run to first base. If the batter fails to run, she can be called out when she enters the dugout. When does it not apply? If there are less than two outs and first base is occupied, a dropped third strike could lead to a sneaky double play. For example, with bases loaded, the catcher could drop the third strike, step on home plate to force out the runner from third base, and throw to first base for a double play. What does it mean for players? As the batter, players should break for first base on any third strike and run until their coach or the umpire lets them know the catcher did catch the ball. Don't wait and see! As a catcher, you should "catch, tag, or throw" with two strikes. A quick catcher will often be able to tag the batter before she starts running, which makes the play much easier and holds other runners. With two outs and bases loaded, the catcher only needs to step on home plate after a dropped third strike, forcing out the runner from third base.  

When a fielder does not have the ball and is not in the act of fielding a batted ball, an obstruction occurs if that fielder impedes the progress of a base runner. The umpire makes an obstruction signal by extending the left arm to the side with a fist. If the impeded runner is put out prior to reaching the next base or a subsequent base that the umpire believes she would have reached if the obstruction had not occurred, then the out does not count and the umpire will put the runner on the base she would have reached absent the obstruction. It is very important to understand that the ball remains live after obstruction occurs and that the runner must try to advance in order to get the base to which she is entitled. When the umpire lowers his or her arm, the runner is no longer protected, either because she has arrived at the base she would have reached or she has failed to try to advance. What does it mean for the player? As fielders, players have a hard time learning to stay out of the way of runners. We see obstruction called in almost every game, and the most common obstruction comes from a fielder standing on a base without the ball when the runner reaches the base. If a fielder is playing directly between two bases when the ball is pitched, she needs to be prepared to move out of the way of a base runner if the ball isn't hit to her. After a ball is hit, if there is no play at her base, then she needs to stay away from the base and out of the runner's way. A runner (as well as the coaches) should look for an obstruction signal by the umpire if she's impeded, and she should continue running to the next base even if a tag is likely. As long as the umpire is making the obstruction signal, the runner is protected in advancing.