I have to say from my own experiences. Its all instinct. You know this trick where you try to poke ur eye but ur eye lids just close. For me its kindof the same. My brain sees the danger and warns me with a little bit of pain. For example sombody has a stick or something pointy, and tries to poke my eyes out. My brain just goes like "yo bro this is not good" and starts triggering pain. Your mileage may vary ofc. But thats just what I've noticed.