apologies, i probably should've written `fmt` there (which does the same thing). `reflow` is a `fmt` clone i wrote, without the knowledge `fmt` existed --- that was a bit embarrassing for a friend to eventually point out, that my idea was already in coreutils :D i use `fmt` now because i figure it's far better tested (i think it's posix), but i still think of the action as a "reflow", and that's also the term helix uses.

'|' 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.