Allowing the fly to sink to the fish's level, the angler makes a retrieve. The fly comes directly at the fish, which suddenly sees its approach. As the small fly get nearer, the fish moves forward to strike, but the tiny fly doesn't flee at the sight of the predator. Instead it continues to come directly toward the fish. Suddenly the fish realizes intuitively that something is wrong(its never happened before), so it flees until it can assess the situation. An opportunity for the angler has been lost.
