notyetagm: After the winning move, 35 f3!, Black is unable to prevent mate in view of the twin threats of 36 h3# and 35 ... xf3 36 xg6#.

When I analyzed the position before White's 35th move, I was struck by the fact that <the powerful White queen on the f7-square creates 3 distinct mating motifs: backrank, Greco, and stalemated king>. By covering the g7- and h7-squares, the White queen traps the Black king on his first rank, allowing <backrank> mating ideas. By covering the g8- and g7-squares (and with the g6-square self-blocked), the White queen traps the Black king on the open h-file, allowing <Greco> mating ideas of swinging a rook to the h-file (f1-f3-h3#). Finally, by covering the g8-, g7- and h7-squares, the White queen <stalemates> the Black king so that <any check is mate>, in particular the White e7-knight going to g6 (e7-g6#). It is the combination of these latter two mating patterns (f1-f3-h3#, e7-g6#) which creates the winning move and forces mate.

notyetagm: When I solved the puzzle, I found that 35 f3! forces mate. But I also found another strong move, 35 f5!. This move wins at least queen for rook, based on the tactical theme of <interference> in the lines 35 f5! xf5 36 gxf5 xe7 37 xe7 or 35 f5! g8(d6) 36 xg6+ xg6 37 xg6 xf5 38 xf5. But I did not analyze this line rigorously once I calculated that 35 f3! was a forced mate.

So 35 f3! forces mate by <deflection> and 35 f5! wins material by <interference>. Very nice tactics to crown an excellent positional performance from Smyslov.