1.Bg5
Black's queen has nowhere to run to, nowhere to hide, caught in the
murderous crossfire of the two white bishops. Instead, Black
stakes his hopes to the following exchange:
1...Bxf3
Black has removed the g5-bishop's protector, the knight at f3, and
simultaneously attacked the white queen. If White recaptures the
bishop with 2.Qxf3 then Black trades queens, 2...Qxf3 3.gxf3 and is
left with the better pawn structure (most of White's pawns are
isolated and doubled) and more importantly - did not lose his queen!
White, however, employs a simple zwischenzug, or in-between move, to
keep his grip on the black queen.
2.Qc1
White moves his own queen from harm's way, while simultaneously adding
back the protection his g5-bishop that Black removed with his first
move.
2...Qxd4 loses to 3.Qc8+ and it's lights out next.
Meanwhile the black queen still has nowhere to
go; Black's best is to get the greatest return possible for his
queen...
2...Qxg5 3.Qxg5 Bd5
...but it's not enough. White has a queen for a knight, and
wins.
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