- insert mode
    - that is a (typed) dash between Ctrl+K and M
    - uppercase M (aside: the K can be upper or lower case)

I started using them a long time ago to type Greek characters, but then I switched to vscode with a vim extension and it didn't support that keybinding so I lost the habit. Might work with the neovim extension now.

edit: Yep, but you have to add "k" to the list of "control characters handled by neovim" in the extension's settings.