need help in understanding computer analysis

i dont understand the queen sacriface as per computer expected move. why nf6 and why queen sacrifice?

pls explain if you could.

White's position is overwhelming.

According to Stockfish, White has a mate in 8 beginning with 1.Bf6+ or 1.Nf6 or 1 Nc7, etc.  I haven't verified that yet with other engines, but it probably is correct.

1...Qg8 simply delays the mate as much as possible.

After 1.Nf6 Re7 or 1.Nf6 Qc7 for example, it's a mate in 5!

1...Qg8 is just a computer move to delay mate for as long as possible.

You can verify this for yourself, of course.  Take the Black side and make any other move other than 1...Qg8.  Your engine will now mate you faster.

This is Houdini's line:

[M8]  1.Nf6 Qg8  2.Nxg8 h5  3.Nf6 Bxf6  4.Bxf6+ Kg8  5.Rg7+ Kh8  6.Qf4 g5  7.Qf5 b6  8.Qh7#

Stockfish's main line is similar:

[M8]  1.Nf6 Qg8 2.Nxg8 h5 3.Nf6 Bxf6 4.Bxf6+ Kg8 5.Bg5 Re7 6.Rxe7 Kh8 7.Qf8+ Rxf8 8.Rxf8# 

thanks for helping...

after nf6, i would move bxf6. then white can move Bxf6+. followed by kg8.  then can u show me the mate move?

Not using a computer, so don't know if this is the "fastest", but it mates Black in 6 moves total.

After 1.Nf6 Bxf6 2.Bxf6+ Kg8, White mates with 3.Rg7+ Kf8 (3...Kh8 4.Rxg6+ Rxf6 5.Qxf6#) 4.Be5+ (Other Bishop retreats along the same diagonal also work) 4...Ke8 (4...Rf6 5.Qxf6+ Ke8 6.Qe7#) 5.Qf7+ Kd8 6.Qd7#

Rg7+ probably and Black can't go to f8 because of Bishop anywhere+ followed by Qf7-d7 mate. Maybe it's too early in the morning but after Kh8 instead I'm not finding anything better than Rxb7+ which prosaically nabs the Black Queen before mating. Like Rxb7+ Kg8 (RxB loses faster) Rxb8 Rxb8 Ba1 or something and it seems the only way to defend f7 is Re7 and then White can play Qf6. There's probably a faster mate with a rook sac or something though. 

Ok so the thing that I missed is rather obvious. lol. I'll leave it stand... it's a weird human analysis mistake. I didn't consider Rxg6+ because the square is defended even though it's not even a sudeo sacrifice. If you actually look at it for a split second you see it's best. Then even thinking there was a faster way to "throw away a rook" and mate after finding a different mate I still didn't look at the best move even enough to notice that's it's not a sac. Computers don't have this problem with moves out of their radar because of how they look.

thanks for answering my doubts. after viewing the solution i understood that my doubts were stupid. anyway, my brain is developing. thanks to all.

Because Rxh7#mate in one; the only defense is Qg8.

Well... it's not accurate to say it is the "only" defense.  There are two other defenses to stop the mate in one.  Either 1...Rxf6 or 1...Bxf6. 

The reason why 1...Qg8 is played is because it's a mate in 4 after 1...Rxf6 and it's a mate in 5 after 1...Bxf6.  Again, 1...Qg8 delays things for as long as possible.

Even Though Black's king is in the square of the pawn, IT CANNOT STOP THE PAWN

WHY?

TRY TO SOLVE