So I don't really shut down. In fact I do the opposite, I go faster, but to the outside world my capabilities look like they're shutting down. First thing is stims will increase a LOT, I'll quickly head into fight, fight or fight territory (if you see what I mean), I'll sound more desperate in questions and requests, these will then fragment till I don't make much sense and I don't get whole sentences out before the next thought is queued to use the vocal chords. ON the inside I'm throwing my entire intelligence at trying to work out what's going on and resolve everything back to peaceful. It might give the appearance of being frozen, but I'm working at 200% of brainpower, but as I'm jumping from thought to thought rapidly, not much is achieved.
I also realise I've had burnouts, and these get bigger each time they happen. If my environment causes lots of the above events (and I mean lots, as in daily or more over the space of weeks and months) in the end something just seems to fracture and I stop responding with any sort of stress or anxiety. Days (weeks) go by with little motivation, little effort, etc. to do anything outside my interests. Just the idea of socialising , looking for works, etc is the mental equivalent of picking up dog poo barehanded. I just shut the door. These have been pretty damaging for my career, but as the sole earner in my house I'm not really in a position to just take a time-out when they're approaching, I just need to trying and mitigate as much as I can.