Hmm..I'm not sure if what I've had are meltdowns, exactly. More like emotional outbursts. But I will explain what I've experienced.
In the past (mostly as a child), I used to lose control and get very angry if something happened that frustrated me or angered me. I would often hit things or hit other people, break things, etc. I can't really remember what else I did during these episodes. But I felt very out of control of myself, and when it was over, I'd feel bad about it. I usually couldn't admit that I felt bad, so I'd pretend that my reaction was justified, and that I'd done whatever it was on purpose.
As an adult, most of the time I react by crying. As far as I know, I have no control over it. If I'm in a disagreement with someone, if someone is not treating me right, or if I just get overwhelmed by a situation, I will start crying. Usually it's out-of-control sobbing, not just tears rolling down my face. It's embarrassing. It even happens sometimes when I'm really angry and trying to control it.
And then I've also had "shutdowns" like what people have described here. When overwhelmed, sometimes my body will suddenly feel very heavy and weak. I become extremely tired, and I usually just go in the bedroom, shut the door, and collapse on my bed. Then I need to be left alone for awhile until I get over it.
I think the main thing that all of these reactions have in common (for me) is the loss of control during extreme emotional states. It literally feels as though I have no control whatsoever over how I react. The crying seems to be the most frequent reaction, and thankfully it's only embarrassing, not harmful.
I should add that a lot of the times, my extreme emotional states are made worse by sensory overload. I will be much more irritable or anxious if sensory issues are bothering me. So it's usually a combination of things. The sensory issues may make it harder for me to control my emotions during stressful times.