All good answers, but here's what really happened.
Rusty had Jim dunk the tip of his rod into the river after Jim made his casts. The water action absorbed the trembling action caused by Jim’s condition, and allowed those flies to float naturally and effectively through the run.
This is a true story, courtesy of Rusty Vorous, legendary Alaska, California, and Montana guide. And one of the best I have fished with in my life.