Four of White's Novotnys in these variations are executed by the rook on b5—only the rook will do in those variations, because the square it comes from, b5, must be vacated for the white knight to deliver mate. In the other two lines, that rook is captured, meaning it cannot execute the Novotny, but also meaning the square vacation is no longer necessary (the white knight can simply capture the black piece on b5). Therefore, another piece can make the Novotny interference in those lines. (There is one other non-thematic defence in the problem: 1...Nd2, threatening a nuisance check on b3. This allows 2.Ra3+ Nb3+ 3.Rxb3#.)