Thin things are hard to paint
A power line can cross an entire photograph and be two pixels wide. Painting it by hand means a long, perfectly steady stroke at high zoom — and a brush wide enough to be forgiving also erases a band of sky either side of it. Wire mode starts from a single tap and does the following for you.
How tracing works
A wire against the sky is a ridge: a narrow line darker (or, for a scratch, lighter) than what is immediately either side of it. From your tap, Wire mode finds the strongest ridge nearby, measures how wide it is, and then walks along it in both directions. At every step it looks a short distance ahead within a narrow cone of its current heading for where the ridge continues, re-centres on it, and smooths its direction so the path follows the gentle sag of a cable rather than wandering into clouds. A short gap — a bird on the wire, a branch crossing it — is coasted over. When the line fades below the sensitivity you set, or the walk reaches the maximum distance, it stops.
The traced path becomes a selection exactly as wide as the wire plus a small margin for its soft edges. That margin is kept tight on purpose, and the usual clean-up that removes tiny islands from a selection is switched off in Wire mode so the thin line is never “tidied” away before it is erased.
Where a line crosses something
Where a wire passes in front of a roof edge, a tree line or the horizon, turn on Structure Repair: it continues straight edges through the gap before the AI fills it, which keeps a roofline straight where the wire used to cross it.