King in Place: Queenside

The "in place" trick again, but now on the queenside! The king is already standing on c1 — exactly where queenside castling leads. So he won't stir, while the rook hops from b1 to d1 — yes, formally "through" the king; in castling that's legal. Beginners don't see a move here at all: "everything's already in place." But you're losing tempo! King — onto the rook b1.