bind <key> stop_irc
Send a SIGTSTP signal to the client, which will return you to your shell. This was an important feature in the days before GNU Screen, when you could only log into a server once, and you might need to step away from IRC and do something else for a moment.
Nobody uses this feature any more, except by mistake (when they accidentally hit ^Z). I bet nobody has intentionally used this feature in 20 years.
If you accidentally invoke this keybinding, type 'fg' to resume. If you stay away for too long, you will get disconnected from all your servers!