Figure 5.3.5.4[White to move]

What does White attack? His knight is ready to take Black’s bishop, except that the bishop is protected by Black’s queen (though not by the pawn on g7, which of course is pinned). White can’t take the queen, but perhaps an attack would drive it off and loosen the bishop. White can threaten it with Be4. This may look scary because the bishop has none of the protection that we usually think necessary before a piece attacks a queen, but White has other advantages to compensate. His knight's threatened capture, NxB+, is a check, so it has the potential to fork any enemy piece that finds its way onto the middle of the board—e4 or d5. In this case, if Black replies to Be4 with QxB his queen gets forked and taken by NxB+. So the bishop’s attack at least is safe.

And Be4 has another virtue: it aims White's bishop at the Black bishop on b7, which also is guarded only by Black’s queen (again, a pattern familiar from our work on skewers). If Black moves his queen over to d6 so as to keep protecting the bishop on f6, the bishop on b7 is lost. Likewise, if Black moves the queen to c7 so as to keep protecting the bishop on b7, the bishop on f6 is lost to NxB.

You can see here that the attacked queen has an impressive ability to move and still defend its protectorate—but not quite impressive enough to allow it to move and still defend two protectorates. The queen, in short, is overworked. But for our purposes here, the point is just that an attack against a queen to drive it away from guard duty becomes a good deal more likely to work when the queen has more than one piece to protect, for then it will be hard for your opponent to find the queen another square where it is safe and still protects both of the vulnerable pieces.