These are mutual knight attacks on dQB pawns. Here's the first example, actually, there are 4 Petroff games in BBNQNRKR :
Actually, I forgot that knights shouldn't block bishops, so there is only 1 Petrov defence.
These are mutual knight attacks on dQB pawns. Here's the first example, actually, there are 4 Petroff games in BBNQNRKR :