johnlspouge: Sunday (Insane): Black to play and win
Material: even. The Rg8 faces the Black Kg1. The Bd3 protects Nc4. The White battery Rd8 and Qd5 pin Bd3 to Qd1, although the move Be2 is available to relieve the pin. The White battery Bb2 and Qd5 have an implicit mate threat at g2, supported by Rg8. White has the nuisance check 19.Qxa4+ Bc6, but because Bc6 threatens Qa4, 19.Qxa4+ does not gain a tempo.
Candidates (18…): Rxg2+, Qxd3, e4, Qd4
18…Qxd3 19.Rd2 (threatening Rxd8#) Qxd2 20.Nxd2, exposing a burden for Nc4
Contrary to initial impression, White is near paralysis. Standard strategy suggests opening the position.
18…e4 (threatening 19…exd3).
Because the nuisance check 19.Qxa4+ does not work, White has two moves: (1) 19.fxe4 and (2) 19.Be2.
(1) 19.fxe4 Qd4 (pinning Rf2, and threatening 20.Bxe4 and 21.Bxd3 or 21.Rxg2+)
To avoid material loss from …Qxd3 and …Qxc4, White has one move:
20.Be2 Qxe4 (threatening 21…Qxe2, 21…Rxd1, or mate, beginning with 21…Rxg2+)
21.Qf1 Qd4 (again pinning Rf2 - this time without Pe4 obstructing Bb7 - and threatening Rxg2+)
22.g3 Nf5, threatening
23…Nxg3 24.hxg3 [else, Nxe2# or Nxf1] Rxg3+ 25.Kh2 Qh4+ 26.Qh3 Qxh3#
There is no defense.
Candidates (19…): e3, Qc5, Qg5
19…Qc5 (threatening 20…Rxd1 and 20…exf3)
20.Qb3 [Nd2 e3] [Qa4+ Bc6 is essentially the same]
Candidates (20…): exf3, Bd4
20…exf3 (threatening 21…Rxg2+)
21.Bxf3 Bxf3 22.Qxf3 Qxc4 23.Qxf6
winning Nc4 for Pf6, with a material advantage in a double-edged but winning position.
I suspect the second line can be improved, but it is sufficient to justify 18…e4, which is a relatively easy candidate to find. I cannot check my analysis of 19.fxe4 with a computer right now, but the response 19.Be2 definitely looks better.