It is Obstruction on the Pitcher?
When obstruction occurs, the umpire shall call or signal “Obstruction.”
(1) 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. Any preceding runners, forced to advance by the award of bases as the penalty for obstruction, shall advance without liability to be put out.
Major League Umpire, Ted Barrett, explains the “protecting one player” rule as it applies to obstruction. This play normally happens up the 1B line as pitcher, first baseman or sometimes catcher converge to field a batted ball in the process, they get tangled up with the batter runner and impede his progress to 1B.