I have one. It's quite impressive.
It's not a tool that gets used much, but at the same time it's a tool that will last forever if you don't let it rust out.
You can get hurt with one, and should be careful.
I almost hate describing it in detail, as a piece of video could show it in seconds. I don't actually have any video of it.
Take the bar off the saw. Put the bar in a vise. File off the burrs, the entire length from both sides.
As you can see from the picture, that says how it works, a threaded lever moves an adjustable bearing toward a stationary bearing. Ya gotta be careful to tension it to the bar gently, give a pull from one end to the other. Fit your chain in to check. Give the threaded lever a quarter turn, give a pull from one end to the other. Fit your chain in to check. Give the threaded lever a quarter turn, give a pull from one end to the other. Fit your chain in to check. If you feel the chains driver links JUST START to grab inside the bar rail, STOP. Trust me. One more 'quarter turn, give a pull from one end to the other' and you'll close the rail too much. They don't sell a rail opening tool, but to let you know, it's called a screwdriver and a hammer and a bunch of unnecessary time trying to open it back up so you can get your chain in.
Fip bar, repeat.
You only would use the bar rail closer when you can flop your chain left/right, meaning the normally vertical-sided bar groove is splayed more like a V. You're squeezing the top rails inward so your chain doesn't flop and cut curves.
This is not a fix for an old worn bar, but it'll extend the life, get you through until your new bar comes in. I do it midlife of the bar during a complete dressing session as it only takes a minute.
It does do what it does really well, I must say. Affordable, too.
