It is a watchdog. It is a feature in which the control software will send a pulse stream through a pin, as if it was moving an axis. Then a microcontroller on the breakout board will see that pulse stream and will know that the signals coming from the controller are valid signals and will then enable the outputs on the board. Otherwise, it will keep all outputs disabled.
All current models of CNC4PC breakout boards have this feature and have a green LED that shows that the outputs are enabled.
It is also important to keep in mind that this feature is controlled by the software, which may not be fail proof, and there have been reported cases of PCs running that have failed, get a blue screen or something like that, and the SCHP remains active. So, this is not a replacement for a hardware disable feature on the system. Note that all our breakout boards have an external enable terminal with the purpose of the user can implement hardware disable on the system. Usually this means connecting the e-stop in parallel with the EN terminal. On boards with a designated e-stop input, it will act as hardware disable for the system.
All our Multifunction Breakout Boards have a relay (Relay3) which reflects the status of the board, active (green LED ON) or Inactive (Green LED Off). You can use that relay to control the main contactor or the main power supply. That way the system will be enabled and powered when the control software is in control (by the SCHP) and no fault or e-stop conditions are active, and the system will be disabled, and power cut of the system on any fault or e-stop condition.