[andrewflnr]

'|' is the command to pipe a selection through a shell command and replace it with the output, so I guess it's just a regular command on their system. You could use any other reflow or reformatting program you had handy. Kakoune in particular is really serious about delegating everything possible to the system.

[llimllib]

I’m curious about the particular program though! On my Mac it doesn’t exist and I didn’t find it in a quick search

[andrewflnr]

Honestly, it might have just been a placeholder name for an arbitrary formatter. I vaguely remember old-school unix had a command for that, but none of them seem to be named exactly "reflow".