US Robotics 33600 User Manual page 71

XON/XOFF
Standard ASCII control characters used to tell an intelligent
device to stop or resume data transmission. In most systems,
typing <Ctrl>-S sends the XOFF character (which tells the
remote device to stop transmitting). Some equipment, including
your modem, interpret
<Ctrl>-Q as XON (which tells the remote device to resume
transmission), others interpret touching any key after <Ctrl>-S
as an XON.
Sportster Winmodem 33600 & x2/56K - User's manual 67