Spotting the kernel of the discovery for Black on the d file should be habitual by now. The challenge is to diagnose and cure the impediments to its success. The first is that the bishop on d6 lacks a good threat; it is aimed at h2, where it could give check, but its path is blocked by its fellow knight. The second problem is that the rook on d8 lacks a good target. If the bishop were vacated from the d-file, the rook would be directed at White’s bishop, which is guarded. Once you clearly understand these obstacles, ideas for getting rid of them at the same time suggest themselves: move the Black knight out of the bishop’s way, and in the process use it to capture the unsuitable target on d3 and cause it to be replaced by something better. Thus NxB, and now if White wants to avoid the loss of the piece he has to recapture QxN. Now both problems have been solved. The Black bishop’s path to h2 is open, and Black’s rook has a good target in the queen on d3.