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.