Fish will certainly hit something moving upstream. I've often watched fish at Bennett, where the water is clear enough to watch your fly and the fish both, and noticed that a trout will often watch a fly just sitting there. They don't take it till I twitch it upstream. They then move up and hit it as though they don't want it to get away.
When you think about it, something appears more lifelike when it is moving upstream. If it wasn't alive it couldn't swim aganist the current. Anything can drift downstream.
Dave