The most profitable fork of all involves the king, because a fork with check leaves the opponent no choice at all. The king MUST deal with the check, and whatever else the fork was attacking is simply lost. No defense, no counterattack, no saving move.
This is why knight checks deserve special attention on every move, yours and your opponent's. Before you park your rook or queen anywhere, glance at the enemy knights and ask: is there one jump that would hit this piece and my king at the same time?
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Black's king on e8 and rook on a8 sit two knight-move hops apart. White's knight is in range.
Fork the king and the rook.
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