I lash out when someone crosses a line: if they start talking about hurting someone that I care about, for example. This actually happened to me once: a roommate seemed serious about hurting someone I cared about. She'd been complaining about this person more and more vehemently for a long time, and I'd tried to get her to see that this person was important to me, but it didn't make any difference. Even so, I was REALLY caught off-guard when she started talking about hurting this person. I ended up actually yelling at the top of my lungs, shaking, and slamming a door. It wasn't a very informative lashing-out, though. All I could muster was something cliched like "I hate you! Get OUT!" Later, I took a more calculated approach to deal with my roommate's intended violence, but I still couldn't hold my own in an actual heated discussion with this person. I had never, ever felt that out of control in my life, but after a cool down period, as soon as the next argument started, my brain went right back to shutting down because my roommate was illogical and often drunk. There was no hope for rational discussion -- only for damage control. (Of course, someone else in my situation might have been able to fight fire with fire and eventually convince her of something, but for the life of me, I could not.)
I lash out less severely when someone shoves me in the middle of a disagreement -- even though it's upsetting, I feel like it's appropriate to have a strong but simple reaction ("Don't touch me!") to a strong but simple insult (the shove). So I feel like I'm still in control of my emotions, and I'm getting my point across.
I also lash out less severely if I feel like I'm in a conversation with someone who knows me well, who I can usually talk to about feelings, but who has misunderstood me and refuses to admit that they have misunderstood me. This type of "lashing out" is more of your typical melodramatic argument; it's just not typical for me.