As we have seen, an attack on a defender of an enemy piece or sensitive square works best when it has something else going for it as well—for example, if the move also gives check or threatens mate, or if the piece being attacked is in an unusually cramped position. Moves that give check and also attack the defender you are trying to remove can also be understood as forks; and lesser but still effective forks are possible in which you attack the defender and some enemy piece other than his king. In cases such as those, where your attack on the guard is part of a double attack, simply moving the defender to a safe place no longer gets your opponent out of trouble; the other piece in the fork remains vulnerable. These sorts of forks also allow you a broader range of targets than usual. Ordinarily a knight forking a rook and a protected bishop is no great threat, since once the rook moves you are faced just with an exchange of minor pieces. But if moving the rook would leave something else loose, the fork becomes interesting after all.
In the position to the left, White sizes up his attacking options and sees that he attacks Black’s knight twice. It also is defended twice. He has no way to capture either defender, but he can attack both of them with the knight fork Nb6. Black is threatened with the loss of his rook, so he must move it to safety; and he can do this while still protecting the a8 bishop by playing Rb8. But now the knight has lost one of its defenders, so White can play BxN. Or White can play NxB, and if Black recaptures RxN then both defenders of Black’s knight are gone, enabling it to be taken for free next move. White wins a piece in any event. Observe how the fork gave Black a narrower set of choices than we saw in cases where a guard was attacked without more. The trouble created by a fork cannot be evaded by just moving the attacked piece, because there are two of them. And notice that while a knight fork of a rook and a protected bishop usually would be of little interest, it is another matter when capturing the bishop would leave an additional target loose for the taking.