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.

