Why is the move Ng3+ brilliant?
1kb4r/2q2p2/pp4r1/2bPp3/2p1P2p/2P2Qn1/P1B2NPP/2B1RR1K w k - 0 1
White only has three possible moves here.
Qxg3 is obviously bad.
After Kg1, Black could just play Nxf1 with a sizable advantage - but there's actually a better follow-up. Bg4 traps the queen since the f2 knight is now pinned.
And after hxg3, the recapture hxg3+ will win the knight back and more.
[FEN "1kb4r/2q2p2/pp4r1/2bPp3/2p1P2p/2P2Qn1/P1B2NPP/2B1RR1K w - - 0 1"]
1. hxg3 (1. Kg1 Bg4 (1... Nxf1 2. Kxf1)) 1... hxg3+ 2. Kg1 (2. Nh3 Bxh3 3. Be3 (3. gxh3 Rxh3+ 4. Kg2 Rh2#) 3... Bg4+ 4. Kg1 Bxf3) 2... gxf2+ 3. Rxf2