The piece sac. line 4...g4 5.Bxf4 gxf3 6.Qxf3 gives good compensation for the sacrificed piece.
Helps to hold on to the center. If g4 then c3! sacrificing a piece with good attacking chances.
Black is planning to trade as much pieces as possible to transfer into an endgame with a pawn up.
I give a ! to this move because it's the only way to get an attack in this position. White is opening up the position in order to kill black on the kingside.
Doing what white wants to do. Better was to close things up, forcing white to sacrifice a piece to open lines with: 12...g4 13.Ne1 f3
This knight is immune due to 13...hxg5 Bxf7+ winning the knight on g6.
Using the black pawn as a shelter.
Black could have played 16...d5! 17.Qh5 (17.Bxd5? Qxd5!)
Now guess the move that Alekhine managed to "see"
Pinning his own bishop to the king allowing a nice blow from white.
19 Bf6! 1-0
The only way to avoid mate is by giving up the queen with Qxe4+ so black resigned. A nice crush by Alekhine, without even looking at the chessboard!
Hope you enjoyed! This is my first analysis so please leave your comments!