Another defensive study. Black has a problem: his knight on c6 is pinned, and is attacked twice and guarded just once. He is imagining 1. BxN, b7xB; 2. Qxc6+, forking king and rook. So Black thinks about moving his c8 bishop off the back rank to d7, shoring up the protection of the knight. It's a blunder. Remember to ask whether a move you consider will leave anything unprotected. This happened in the previous position when a piece was moved to a square that had no protection. That isn’t a problem here, since the bishop will be safe on d7. But you also want to look at any lines a move affects. The point is the knight on d5. It's protected by the queen. The knight’s protection is cut off if Black plays Bd7; it becomes loose.

Once you see this, it isn't enough to ask just whether the knight now is under attack. Loose pieces can be taken for free by forks. Is your king also exposed to attack? Do you have any other loose pieces? Where is your opponent’s queen? Here your study of those questions would show that you already have an unprotected bishop at c5. So Bd7 would leave two loose pieces right next to each other. And White’s queen is close at hand: on a4, ready to play Qc4 and win one of those two pieces a move later.