Windows issues a BREAK over the serial lines of communication when you cycle the power on the computer. The 6K and 6000 Series stand-alone products take this BREAK and kill any program that is running.
The 6K and 6000 Series products have an undocumented command, ENBRK, that allows the user to control this.
ENBRK1 (default) --Program will stop on BREAK.
ENBRK0 --Program will not stop when a BREAK is issued.