The Bishop fork
In diagram1 the bishop is attacking the two white rooks at the same time.
White cannot save both of them. The best he can do is to move his rook from a6 to a8 (diagram2)
Diagram 1 Diagram2
Now when black captures one of the rooks (diagram3) white can capture the bishop with the other .(diagram4).
Since a rook is worth 5 pawns and a bishop is worth only 3 pawns black has done best out of this exchange but white has got something in exchange for his rook.
Diagram 3 Diagram 4
