When there is a collision between a runner and a fielder who is in clear possession of the ball, the umpire shall judge: If the defensive player blocks the base (plate) or base line with clear possession of the ball, the runner may slide into to make contact with the fielder as long as the runner is making a legitimate attempt to reach the base (plate). There are additional nuances to this rule.
[OBR 6.01i: A runner attempting to score may not deviate from his direct pathway to the plate in order to initiate contact with the catcher (or other player covering home plate), or otherwise initiate an avoidable collision. If, in the judgment of the umpire, a runner attempting to score initiates contact with the catcher (or other player covering home plate) in such a manner, the umpire shall declare the runner out (regardless of whether the player covering home plate maintains possession of the ball). In such circumstances, the umpire shall call the ball dead, and all other base runners shall return to the last base touched at the time of the collision. If the runner slides into the plate in an appropriate manner, he shall not be adjudged to have violated this rule.]