Why would you slip line, or not slip line, during a reach mend?

Slip line to hold the fly at distance while the rod reaches; do not slip when you want a fixed amount of line and a shorter, precise landing.

Slip line when the reach must not steal distance from the cast; do not slip when you want a fixed amount of line and a shorter, precise landing.

No slip. The reach consumes approximately 10 ft of the line’s length and halts the fly’s forward movement, so a 40 ft cast lands short — which is the point. It casts a fixed amount of line. In the guide’s example — a river flowing right to left, target 30 ft away — cast 40 ft and reach right, and the fly lands in the narrow feeding lane above the fish. It is excellent for dry fly presentation at short distances.
With slip. Line slips through the guides during the reach, so the rod reaches without pulling the fly back. You do not have to cast past the target, which would spook fish or snag a grassy bank.

Pull off enough extra line for the fly to still land near 40 ft after slipping; start with 10 extra feet.

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