tl;dr The overheat mechanic from the BattleTech / MechWarrior games is a really good metaphor for being triggered when you have Post-Traumatic Stress Disorder.
BattleTech is a board game played with miniatures which represent BattleMechs, which are basically big walking tanks. It has a whole slew of spinoff games and novels, including the MechWarrior PC and console games, which most people are probably more likely to have heard of.
Most of the games let you customize your mech. The big limiting factor, though, that keeps you from slapping on as many weapons as you want and mashing the fire button, is the fact that firing weapons heats up your mech and can cause it to shut down.
Someone I knew once put twelve fire-linked extended-range large lasers on a single mech chassis, just to see what would happen. As he put it, the "fire" button doubled as the ejection seat switch.
It's an easy mechanic to get the hang of, because it's ludicrously detailed but all the details make intuitive sense. Using energy weapons overheats you faster than using projectile weapons. Operating a mech on a desert planet generates more heat than driving it through the tundra. Standing hip-deep in water while firing weapons helps cool your mech down. And if you want, when building your mech you can skimp on armour plating or advanced electronics and put in a bunch of heat sinks, which work exactly like you'd expect.
How this relates to PTSD
Having your PTSD triggered is not the same as "choosing to be offended," or throwing a fit because you aren't getting your way. (In my experience, the people most likely to accuse others of doing that are narcissists, who all seem to think it's a legit debate tactic or parenting method when it's them doing it.)
PTSD triggers are called triggers because they're reflexive responses, like the ones most people have upon seeing a jump scare or being tapped on the knee with a hammer. They can immediately put someone into "fight or flight" mode. But when you've been socialized not to do either (like in my case), or you "heat up" too much or too rapidly, they can make you just freeze up instead.
Just last night, someone said something that created a huge spike of "heat" during an FFXIV raid, when I already wasn't doing too well, and I shut down just like a BattleMech that overheated. I was watching the fight going on around me, and I couldn't do anything about it. I couldn't move. I couldn't talk. I could barely even think. I felt like a deer in their headlights.
I'm having trouble even writing this, because I remember how I felt and I freeze up again.
What you can do
It's not hard to explain how to deal with someone who has PTSD. There are really common triggers to avoid, like rape jokes and anything sexist / cissexist / homophobic. There are signs to look out for, like that deer-in-the-headlights look and someone freezing up and being unable to talk, or suddenly becoming really defensive. And there are things you can do, like say "I'm sorry if that was a trigger" and "it's okay if you need to take a break."
You don't have to understand how it feels to be triggered if you can just think of a mech shutting down, and let the person cool off and get back to "all systems nominal."