Scan for checks with Black’s rooks and you find two: Rh1 (losing the rook to KxR), and Rg3. Ask whether Rg3 attacks anything else, and find the White knight at g5, which is loose (so you would have been looking for a way to take it anyway). Does Rg3+ lose the rook to f2xR? No; the pawn is pinned by Black’s bishop at c5, so its protection of g3 is illusory. Rg3+ thus wins the knight. This is an example of a rook fork requiring a horizontal move; it may be a little harder than the other type to see at first, as noted earlier.