Fish4fun wrote:
If you’re dead drifting you’re moving downstream with the current with no drag on the fly. When you’re swinging the drag is pulling the fly across the current as it moves down stream.
I would add that to accomplish a dead drift you should cast up stream at about 45 degrees across current. You also need the mend your line so that it doesn't create drag on the fly. This goes for both dry flies and nymphing.