Source : Free On-Line Dictionary of Computing

software handshaking
     
         The transmission of extra data on a channel
        in order to control the device sending data in the other
        direction on that channel.  For an {EIA-232} connection, this
        means sending {Control-S} and {Control-Q} characters to stop
        and start transmission.
     
        Since software handshaking requires the transmission and
        processing of extra data it can be less efficient than
        {hardware handshaking}.
     
        (1996-10-16)