Chess>Combinations>DoublePinQueenBishopBoxMate

White To Play

1) Re1     Qxe1

This is a Pin Guard Attack." The Rook has "Impunity," it cannot be captured by the Queen as we see in this example. Not only that but if the Queen moves away it will either be captured or the King will be mated in a manner similar to this example. If any other possible move is made, the Queen will be captured by the Rook. Also you will note that Black's Knight is caught in a "Double Pin."

2) Qxf6+   kg8

This is an "Exchange Sacrifice" since Black gets the Rook and White only gets a Knight in return. White "Captures with Check." The Black King has "Only One Move," "Back to the Box."

3) Bh6      Qd7

"The Sword of Damocles" is now in place.

4) Qf7#