I didn't read all the posts, so if I repeat anything, I apologize.
I just recently researched this. They say (and many seem to agree)... First off... there's a difference between meltdowns and temper tantrums. Yes, adults have temper tantrums too. The easiest way to tell which is which is temper tantrums, the person is trying to get your attention, they are aware of their surroundings and take a certain level of precautions to not get hurt, and it feels like they are in control of the situation like they are manipulating the world around them. Meltdowns, the person has no concept of his/her surroundings and it feels like nobody is in control.
Every meltdown has phases. Before the meltdown, things happen to lead up to it. Usually it goes completely unnoticed, and then all the sudden, snap... point of no return meltdown. The thing though is to intervene before the meltdown to help avoid it. Some things that seem to play a certain role are fatigue, sensory overload, series of bad luck, etc. I think every Aspie has different meltdown triggers, so it really is a matter of observation and trial and error. When I feel sensory overload, I walk away from the situation before it turns into a meltdown.
During meltdowns, the important thing is to do your best to provide a safe environment for it. Remove breakables (especially valuable ones), and try to remove other people or the person with the meltdown from other people in order to avoid other people getting hurt. Make sure you don't get hurt in the process as well. Otherwise, just let it ride itself out. Don't bother or badger the person with autism. That includes talking to them in the spirit of "counsel, advice, or even comfort." All they need is a minute to collect themselves and kinda process all the information that is happening. They can't do that with someone bothering them.
Afterwards, it is common for someone to experience intense fatigue. So a nap is a good thing for the Aspie.
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"In the room the women come and go talking of Michelangelo." J. Alfred Prufrock