Desperation. There were several alternative possibilities. For example, 18. ... Rc7. This would have been met by NxBb7, RxNb7; NxNd5, PxNd5; after which White has an embarrassment of riches including piling up rooks on the d- and e-files, and simply taking Qxd5. 18. ... Bf8 fails to BxNd7+, QxBd7 (forced); QxQ#. Relatively better was perhaps 18. ... Nf6 or ... Nb6 to shore up d7. White would have continued NxNd7, NxNd7; Rd1 etc.
19. Qd6xc5 Nd5xc3 20. bxc3 Qd8e7 21. Ra1d1
Crunch: the d7 knight is doomed. If 21. ... Bd5 to block the R, then c4. If 21. ... QxQ, then BxN+, K-any; BxQc5. As Black was already exchange down, he did the honorable thing. 1-0.
