Help needed to understand a move.

2. ... gxf6 3. Bh6+ Kf7 (Ke8 and Kg8 both lead to mate after 4. Nxf6+ Kf7 Ng5#) 4. Nfg5+ (notice the pin on the black f-pawn) Kg6 5. Rxf6+ Kh5 6. Be2+ Kh4 7. g3+ Bxg3 8. hxg3#.

That's the continuation I believe.

Bh6 is the key.

Ok, I think I've got it.

Mate in 7

short answer is all lines lead to mate like the 2 lines shown. also if say Kf7 leads to mate as well at move 19 i can show u if u would like:)

I was too slow. Puzzle solution is the same as the line given be efour above. I think I have covered all possible variations.

Practical answer is Nfg5 is completely winning too, because black's position is absolutely junk, so it was good to play the move you could see the end of.

Of course the computer doesn't care about that sort of thing and finds a forced mate.  Not a practical sequence, but it's nice to see possibilities in a position sometimes.

"This is a blunder by Black. In the current position the game is scored (18.7)"

18.7 advantage with even material? That's huge. It means white's going to get a lot of material. The move it makes is because mate is inevitable or loss of material.

If gxf6, white has a

guaranteed checkmate.

2... gxf6

3. Bh6+ Kf7

4. Nfg5+ Kg6

5. Rxf6+ Kh5

6. Be2+ Kh4

7. g3+ Bxg3

8. hxg3#

Thank you everyone for your help. Clear and illuminating. I look forward to the day when I have the same vision as everyone who posted with lines and insights. 

u do already mepearson u just have to dig deep into ur chess knowledge and polish up