Usinng Aux 5 or 6 to slow feed rate down in corners or other creative uses?

Could some sort of sensor widget be attached to aux 5 or 6 to slow the federate cutting speed for more accuracy in the corners.

Another idea I had was an optical type sensor underneath the sled so if the sled were to ever come off, the sensor would see more light and could notify the arudino to shut down?

I have not seen anyone use Aux 5 or 6 (could of easily missed the post) What are some practical uses of these additional sensory inputs?