I am a bit late to the party, but I still wanted to give in my impression because I didn't see it mentioned by anyone else yet.

I think for me, it feels extremely overwhelming. I feel myself shutting down, speaking becomes basically impossible. Luckily I can still write on my phone, albeit with huge effort, so I can somehow communicate with other people. I only meltdown when I'm in a safe space, but I also remove myself when I notice it gets too much. This feeling of "it becomes too much" is difficult to describe, but it feels like an inner shaking and rumbling. A bit like a volcano. For me personally, this is an unmistakable sign that I need to get out of a situation right now. I do not know if I'm able to go beyond this rumbling sign and not meltdown, and I'm not really keen on trying so. I presume that it would probably depend on the situation, on my resources and the context.

I think the most surprising thing about it is that some very deep part of me likes the feeling of a meltdown. It is difficult to describe why, especially because a meltdown isn't fun or relaxing in any way. For me however it feels like a huge power which I am absolutely not able to control in any way, so I can just let myself go in some sense. I don't have to try and function anymore, nor do I have to behave in a certain way; instead I can just let myself happen. It is a very deep unity with myself and my needs. That doesn't mean at all that I try to have meltdowns; usually after one my day is done for, I'm basically just existing, my head hurts and I feel absolutely exhausted.

In terms of communicating, I'd advise you to prepare some cards beforehand with phrases you might need (e.g. "I need a dark room", "Please stay here", " Could you bring me some water" etc.) Just common things you might want to say. Then you won't have to struggle with saying or writing, but can just pick a card.

I would love to finish this comment with a reminder that the way you, I and we experience and feel the world is valid and okay. Having a meltdown and e.g. struggling to communicate during one isn't anything we need to feel bad and ashamed about.