The reach moves the rod, and without a little slip the fly follows the rod. Letting line slide through the line hand keeps the fly on target while the rod travels out to place the mend.

The sequence is simple: pick up and lay down, stop the rod, then reach in the direction you want the slack. That reach is an aerial mend. A small amount of line has to slide through your hand during it, and that is what keeps the cast accurate instead of dragging the fly short of the target.

How much you allow is not a fixed number. It has to suit the reach and the layout you want the line to land in: let too little go and the fly is pulled off its target; let out what the reach needs and you keep both the distance and the mend.

This is a fishing cast, not a distance trick. The point of the slack is a natural drift, so the fly keeps moving down the current while the line lies where you put it.

Source

From the Slack-line casts and reach-mend demonstration; the CI Study Guide (2025) covers the reach mend in Task 4; terms follow the FFI Fly Casting Definitions (Mends, Slipping Line).