Software flow control makes use of two reserved control characters, XON (Control-Q or 0x11) and XOFF (Control-S or 0x13), to ask the sender on either side of a connection to halt transmission temporarily as a means of flow control. Whenever the receiver sends an XOFF character during data transfer, the transmitter will stop transmission. It will resume transmission only after receiving an XON character.
Serial currently only supports this feature on devices where it is supported in hardware- which currently includes FTDI devices. We will be supporting this on more devices very soon.
At any time you may manually send XON/XOFF characters to a remote device using the Control-Q (for XON) or Control-S (for XOFF) key sequences.