What is the "extra perimeters on overhangs" setting actually good for?
PrusaSlicer has an experimental setting under Print Settings > Layers and Perimeters > Quality, called "Extra perimeters on overhangs".
The popup that appears when hovering over the setting label links to https://help.prusa3d.com/article/layers-and-perimeters_1748#extra-perimeters-on-overhangs - but that section doesn't exist yet. It also explains "Detect areas where bridges cannot be anchored, and fill them with extra perimeter paths. These paths are anchored to the nearby overhang area when possible."
I have to wonder what the use case would be in an actual print job.
Here's how a bridge normally looks in PrusaSlicer without that setting, for a bridge attached to a thin wall:
Everything above looks fine. Now let's enable "extra perimeters on overhangs" and we get this:
I didn't realize that setting was enabled when I started a 4-hour print job. As you might surmise from the image above, the bridge is made of loops, these loops are unsupported, and they are going to collapse.
Sure enough....
I have to wonder, when would I need this setting? Even if the wall was just 1 perimeter thick, I still don't see how this feature would benefit me.
RE: What is the "extra perimeters on overhangs" setting actually good for?
In my experience, it can help when printing features on supports. It starts on the innermost perimeter, as next perimeters shrink due to cooling, it stops them from separating from the supports. It may also help on steep overhangs, so infill doesn't print into open area.
Looks like in your case, it tried to replace bridge, which isn't what it should do, but since it's an experimental settings, it has some bugs.