A double play, where two outs are made on one at-bat, can be a momentum gainer or killer. Runners often do whatever they can to break one up, including running into the fielder. Baseball has a few rules in place so the runner doesn't have an unfair advantage and midfielders have a better chance to maintain their health.
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It's the runner's job to attempt to break up the double play. However, he can only do it legally if he stays within the basepath. The runner is out if he slides more than the length of his arm to either side of the base. If he interferes with the throw in any way, the batter is out as well. Though rare, the same rules apply to a play at third base.
You have to hit the ground to break up a double play, and runners have Hal McRae to thank for that rule. McRae gained notoriety as one of the more fiery players in baseball during his run with the Cincinnati Reds and Kansas City Royals. He had a habit of charging the infielder, sometimes lowering his shoulder into the man. It caused enough pain that baseball put in the mandatory sliding rule. The so-called McRae rule means it's automatic interference if the runner doesn't slide and makes contact with a fielder attempting a double play.