A dead-drift dies the moment your fly line lands on water moving at a different speed than your fly. The line bellies out, comes tight, and yanks the fly off its natural path. That tug is drag, and a mend is how you erase it.
A mend is flipping the belly of your fly line upstream (or downstream) without moving the fly. Done right, it resets the line so the current can't grab it and drag your fly off the seam it's drifting.
The two you'll use:
- Upstream mend. The everyday one. When the current between you and your fly is faster than the water your fly is sitting in, flip the belly of line upstream. That hands the fly slack to keep drifting at the speed of its own current.
- Downstream mend. Less common. For when your fly is in faster water than your line - roll the belly downstream so the fly isn't held back.
The motion is a low, rolling lift of the rod tip that picks up only the line and sets it back down. Not a yank. If the fly twitches or skates, you mended too hard.
Mend early and mend often. Throw a fresh mend the instant the line lands, then small mends through the drift. Each one buys another couple feet of clean float before drag creeps back.
The next step up is reach casting: mending in the air, before the line ever lands.