In the variation White chose for this game, Black has to worry about 3 potential threats: a) White establishes an unassailable Knight on c4; b) White pushes through the e4-e5 break; c) White swaps off the Black Bg7.

Pirc prevents the first by simply sacrificing a pawn that square and gives himself a nice dark-square bind; neutralizes the second threat so that it causes no harm; and deals with the third threat by making his own Knight the most powerful minor on the board. Indeed, White must soon give up his own dark-square Bishop to stop the horse's one-steed stampede.