Try to keep the final play at least 20 feet from the back of the boat. When the fish surges on the bait, signaling the switch, you don't want him up in the prop wash. While it may not spook him entirely, the wash can confuse him in the critical seconds. He may swim under the boat, away from the fly, and the teased-up momentum is lessened or lost. The longer the fly is in the water after the switch, the less chance the fish will eat it.