If you start a shell from another shell, you can suspend the inner one. Say when using `su`, and wanting to switch back to the regular user for a moment:
user$ su
Password: ...
root# do something
root# suspend
user$ do something as the ordinary user again
user$ fg
root# ...
(If you do that, don't forget the privileged shell open in the background...)
Similarly, if you escape to a shell from some other program (the `!` command in e.g. `less`), you can still suspend the shell. But I wouldn't expect many other programs to handle it nicely when they launch a subprocess, which then suspends itself.