We saw this position in the chapter on capturing the guard (see 5.1.2.5.). White is considering Nxe4, which looks plausible because the pawn is protected once and attacked twice. But in the earlier chapter we saw that Black can reply QxQ+, removing the knight’s guard; after White recaptures, Black has NxN and wins a piece. Now you can see another refutation for Black as well. After White plays Nxe4, his knight is guarded by his queen; that is what prevents NxN. Look at what else the White queen protects, or look for any checks Black can give, or look for any intriguing visual pattern; the result by any route is Qxc3+—a triangular queen fork that wins the rook on a1. So after White starts with Nxe4, Black goes ahead with NxN, inviting White to play QxN. If he does, he leaves the forking square loose and Black plays Qxc3+, gaining a rook. White overworked his queen, using it to guard his knight and a forking square.

We revisit an earlier point: if you venture a piece out onto the board, perhaps to grab a pawn, and expect it to be safe because one of your pieces will protect it, you must pause to ask not only if the guardian can be taken, but also what else it is supposed to protect—what pieces, and what squares.