This read-write enumeration enables or disables the fallback speed mechanism on the transmitter, which is only meaningful when the unit is set for DCE internal clocking; the actual fallback rate is computed automatically by the unit rather than set manually. An administrator toggles this to activate automatic speed reduction as a resilience measure for most user protocols other than ASYNC and PASSTHRU. For example, enabling this on a DCE-clocked port lets the unit automatically drop to a lower, more reliable speed rather than losing the connection outright when line conditions degrade.