Colin McNab is playing White against Colin Crouch. There is an immediate priority to analyse 15. Nd5+. An extremely lazy player would now say to himself: 'After 15. Nd5+ cxd5 16. cxd5, Black has either 16 . . . Bxh3 or 16 . . . Nxd5 17. Bxd5 Bxh3. There may be a few problems after that, but one of them must be OK.'
An averagely lazy player will see that 16 . . . Bxh3 17. d6+ Kc8 18. Bxh3 leaves Black in a terrible mess with Bxd7+, Rxf7 and e6 all menacingly in the air.
Colin Crouch probably saw all that, and looked at 16 . . . Nxd5 17. Bxd5 Bxh3 18. Rxf7 Rhg8 before deciding it was fine for Black, since 19. e6 allows Bxb2. McNab, however, saw further. In deciding to play 15. Nd5+, he needed to continue the analysis with 19. e6 Bxb2 20. Rxd7+ Kb6 21. Rxb7+ Ka6 and realise that Black was in the most awful mess after 22. e7. The game ended 22 . . . Rg7 23. Rb4] and Black resigned. If he moves the attacked rook from a8, there follows 24. Bb7+ Ka5 25. Rb5 mate.