From the 2x25's manual:
"Emergency Stop:
In Packetized Serial mode, the S2 input is configured as an active-low emergency stop. It is pulled high internally, so if this feature isn’t needed,
it can be ignored. If an emergency stop is desired, all the S2 inputs can be tied together. Pulling the S2 input low will cause the driver to
shut down. This should be tied to an emergency stop button if used in a device that could endanger humans."
I didn't see it, but I assume that your µC just sends it a command to restart the controller.