According to Rule 7.06 When obstruction occurs, the umpire shall call or signal “Obstruction.”
(a) If a play is being made on the obstructed runner, or if the batter-runner is obstructed before he touches first base, the ball is dead and all runners shall advance, without liability to be put out, to the bases they would have reached, in the umpire's judgment, if there had been no obstruction. The obstructed runner shall be awarded at least one base beyond the base he had last legally touched before the obstruction.
(a) (1)
He runs more than three feet away from his baseline to avoid being tagged unless his action is to avoid interference with a fielder fielding a batted ball. A runner's baseline is established when the tag attempt occurs and is a straight line from the runner to the base he is attempting to reach safely; or
Any preceding runners, forced to advance by the award of bases as the penalty for obstruction, shall advance without liability to be put out.
Rule 7.06(a) Comment: When a play is being made on an obstructed runner, the umpire shall signal obstruction in the same manner that he calls “Time,” with both hands overhead. The ball is immediately dead when this signal is given; however, should a thrown ball be in flight before the obstruction is called by the umpire, the runners are to be awarded such bases on wild throws as they would have been awarded had not obstruction occurred. On a play where a runner was trapped between second and third and obstructed by the third baseman going into third base while the throw is in flight from the shortstop, if such throw goes into the dugout the obstructed runner is to be awarded home base. Any other runners on base in this situation would also be awarded two bases from the base they last legally touched before obstruction was called.
(b) If no play is being made on the obstructed runner, the play shall proceed until no further action is possible. The umpire shall then call “Time” and impose such penalties, if any, as in his judgment will nullify the act of obstruction.
Rule 7.06(b) Comment: Under 7.06(b) when the ball is not dead on obstruction and an obstructed runner advances beyond the base which, in the umpire's judgment, he would have been awarded because of being obstructed, he does so at his own peril and may be tagged out. This is a judgment call.
NOTE: The catcher, without the ball in his possession, has no right to block the pathway of the runner attempting to score. The base line belongs to the runner and the catcher should be there only when he is fielding a ball or when he already has the ball in his hand.
The key here is that runners would advance to the base they wold have reached "in the umpire's judgement". Since runners routinely score from 2nd base on singles the ump could very well give them two bases, so interfering with the batter is pointless and a good way to get your pitcher hurt.