:control-S: vi. "Stop talking for a second." From the ASCII DC3
or XOFF character (the pronunciation /X-of/ is therefore also
used). Control-S differs from {control-O} in that the person is
asked to stop talking (perhaps because you are on the phone) but
will be allowed to continue when you're ready to listen to him --
as opposed to control-O, which has more of the meaning of
"Shut up." Considered silly.
-- The AI Hackers Dictionary