A. H. E. Wood states in "Greased Line Fishing for Salmon [and Steelhead] by Jock Scott Introduction by Bill Mc Millan" "Down and to the bank." Fish headed down stream turn upstream. very logical, when the rod tip is up then lowered horizontal the river bank to point downstream the fly line gets a bit of slack, a downstream curve is formed, and the tug from the line on the fish changes direction. a fish will try to pull away from the direction of the teather so then the fish thinks he needs to go updtream to escape and as tigfly has noted the hook pulls into the mouth.