Interference - Rule 2.00(a)(b)
The definition as stated in Rule 2.00 is:
It should be noted that (b) above is the only defensive interference. Hindering the runner by the defense is obstruction.
Interference is one of the toughest calls an umpire has to make. It is based solely on the umpire's judgement. To make a good judgement as to whether interference occurred, the umpire must know the definition as stated in the rules so that it can be recognized when it occurs. After interference is called, the proper rule must be applied.
How is the rule interpreted? The key is to focus on the phrase "interferes with, obstructs, impedes, hinders or confuses." Those words cover a lot of actions. The umpire, after witnessing an act by the offense, must ask himself the following question: "Did the offense interfere with, obstruct, impede, hinder or confuse the fielder attempting to make the play?" If the answer is yes, interference should be called. The call must be made as soon as possible. When interference is called the ball is immediately dead and no runners may advance beyond the base they held at the time of the interference. The umpire must know where all the runners were at the time of the call.
Some interference calls are easy.
Example: If a batted ball hits a runner, he is out. No judgement of intent is required unless a deflected ball hits him, in which case the umpire must decide if he intended to be hit in an effort to interfere, obstruct, impede, hinder or confuse the defense.
Example: A runner must avoid a fielder attempting to field a batted ball. If he does not he is guilty. This is a fairly easy call.
Difficult calls are the one involving thrown balls. Interference with a thrown ball must be judged as an intentional act. If a runner is hit by a thrown ball while running the bases, he is not out unless the umpire judges that the runner intentionally interfered, obstructed, hindered or confused the defense attempting to make a play.
Some examples of interference are: