Figure[Black to move]

Do you see a mating idea for Black? The clue is the open h-file occupied by his rook. If the rook could land on h1 with protection—the traditional provider of it would be a bishop; we have seen bishop-and-rook mates like this before—it would be mate. As it happens Black has a bishop aimed that way on the long diagonal, but two things lie in its way: the White pawn on d5 and White bishop on g2. Without those pieces, White would be unable to avoid mate. The bishop protects the pawn, and Black attacks the pawn twice, so a method of clearing the line comes into view: Black plays Qxd5 (threatening QxB#); White replies BxQ; Black plays BxB and now is poised to play Rh1#. Indeed, nothing White does can prevent it; he only has useless interpositions with his bishop or queen on the h-file or with his queen on the long diagonal.

What does all this have to do with pins? It is a pin that stops White from effectively blocking the mate with f2-f3. White’s f-pawn is pinned by the bishop on c5, and seeing this might have encouraged the whole thought process above: one consequence of a pin on a pawn is that it can’t step forward to block anything; diagonals on which it might have interposed are left free, giving you more liberty to play on them and arrange attacks using queens and bishops there.