We have seen that the queen can make its sacrifice on f7 or a comparable square by swooping in horizontally or vertically. Here it follows a diagonal path. White’s battery of queen and bishop alone here would not be enough to mate; the important thing to see is the combination of those pieces with the rook on the open a-file. Qxf7+ forces RxQ, and now again the rook is pinned. Ra8+ mates after Black makes useless interpositions (that nevertheless are good to notice) with his queen and bishop.