12...Bf5

At first glance, 12...Bf5 looks like a blunder; the Bishop is under attack, but undefended. A second glance shows that the Bishop is under attack by the only piece defending the Knight on e2, and that Knight is attacked by the Rook on e8.

The Bishop move, then, is a complicated offer to exchange minor pieces. White might get an impressive piece on f5, but Black will get a Rook on its seventh rank.

Next move: 13.Bxf5