1.Nxd4 Qxd4
White has attracted the black queen to the same diagonal as the
undefended g7-square, which is shielding the rook on h8, and the rook
has nowhere to run to, nowhere to hide!
2.Bb2
The black queen is threatened, but the real target is the g7-pawn &
the h8-rook. This is called a skewer, where the real
target is not the more valuable piece immediately targeted, but a
piece of (usually) lesser value in line (diagonally, or along a rank
or file) behind it. Once the queen moves away, White picks up a
load of material.
2...Qd5 3.Bxg7
Black cannot save the rook.
Note that Black didn't try
to protect the g7-pawn by 2...Qg4, because White would just trade
queens (3.Qxg4 Bxg4) [diagram]
Analysis Diagram: after 3...Bxg4
...and then win the pawn & rook anyway (with 4.Bxg7). Since
Black is going to lose the same amount of material in both lines, he
chooses to keep the queens on the board.
This is an
important point to remember: the general rule is that when you are
behind in material, you might want to trade pawns, but not pieces.
The reason is that you want to keep as many of your pieces on the
board to fight with, hoping that your opponent blunders and lets you
back in the game.
Conversely, when you are ahead in
material, seek to exchange pieces but not pawns. If you are one
piece ahead, the advantage is much greater when it's 2-1 (twice the
material) then when it's 5-4 (only one-quarter more material.)
So you exchange pieces to increase the effect of your material
advantage, but keep the pawns on so you can promote one to a queen
later in the game.
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