Meltdowns for me feel like a gradual build up to a severe fight-or-flight response. When it first starts I just want to be alone, but if someone/something continues to aggravate me it can quickly escalate. As this feeling builds, the rational part of my brain starts shutting down and I start feeling like a caged animal who needs to escape by any means necessary.
I can usually calm myself down but there have been a few times a full on meltdown happened. The height of a meltdown feels like a brief double dose of adrenaline. I feel no pain, I feel no empathy or remorse and want only to clear my immediate area of the people and things that are irritating me. In that state of mind it's like those people and things are attackers and I've either got to get the fuck out or start fighting back with everything I've got.
In those moments my mind functions on autopilot with a white-hot clarity of purpose. I will say the most immediately damaging, emotionally crippling things I can think of to quickly end the situation. If that doesn't stop the situation or worse escalates the verbal abuse from the other side, there's a point where the human part of my mind completely turns off and my brain stem is in control. I become like an ape trying to evade or frighten away a predator.
If it's a person who is bringing it on I always warn them and try to get away. If it's in the heat of an argument and the other person just can't let it go for a few minutes things can get seriously ugly. I always regret whatever I say or do afterward, but that doesn't change anything. I know what's going to happen going in, but that doesn't change anything. My only way to control that situation is to momentarily leave to regain my composure and hope the other person isn't too stubborn to let me be alone for a while.
To be clear I've never hit anyone, not since playground scraps in school anyway. It's usually walls and fragile glass things that are the focus of my outbursts with my own body taking the worst of it.