When sliding into a base in an attempt to break up a double play, a runner has to make a "bona fide slide." Such is defined as the runner making contact with the ground before reaching the base, being able to reach the base with a hand or foot, being able to remain on the base at the completion of the slide (except at home plate) and not changing his path for the purpose of initiating contact with a fielder. The slide rule prohibits runners from using a "roll block" or attempting to initiate contact with the fielder by elevating and kicking his leg above the fielder's knee, throwing his arm or his upper body or grabbing the fielder. When a violation of the slide rule occurs, the offending runner and the batter-runner will be called out.
Accidental contact can occur in the course of a permissible slide, and a runner will not be called for interference if contact is caused by a fielder being in the runner's legal pathway to the base.
History of the rule
Amendments to the sliding rules were implemented after a 2015 season in which a number of middle infielders were injured by sliding baserunners while covering second base.
Previously, runners were given a good deal of leeway when sliding into a base in an attempt to break up a double play. Even on plays when they made contact with a fielder, runners were rarely called for interference provided they were in reach of the base at any point in the course of their slides.
Watch: The Royals challenge and are awarded a double play after Jake Marisnick commits a slide-rule violation at second base.