It happens instantly, automatically, and from an unexamined place in your mind. It shows up in a sudden shift from open, happy, creative, and conversational to defensive, and, depending on your typical reaction, engaged in some form of fight, flight, freeze, or appease. I have heard it called by a number of terms. For me, nothing says it more directly than the “kill switch” that instantly cuts off your aliveness and puts you into survival behavior. Apparently all human beings have a kill switch. It gets set off by all kinds of circumstances and interactions. The common thread is an unwitting reminder of what you believe to be wrong with you. It not only reminds you, the "incoming" matches the story you have been telling yourself most all of your life and serves to confirm your limiting belief. At the level of your limbic system (old brain), it occurs as a threat to your survival. Since the limbic system can't tell the difference between a physical threat and a threat in language, the event has the same impact on your body chemistry and aliveness.
This underlying, deep-seated belief and all the stories you have collected up to this point in your life that seemingly validate this belief are being talked about more and more by leading speakers and authors. Dr. Brene Brown refers to it as your "shame." I've also heard it called your “Original Wound.” I simply refer to it as that first big rotten thing you said about yourself, very early in life, that you fervently believe to this day. It is some version of "I suck." Each of us has our own very personal version burned into our mental software.
From my observation of clients over thousands of hours of one-to-one work, this kill switch, or the invoking of your shame, whether realized at the moment or not, will dramatically limit your effectiveness and your ability to experience, love, health, happiness, and full self-expression. Other than that, no problem.
I have seen more than one marriage destroyed because one or the other has a kill switch that says I'm not lovable. I've seen careers severely limited or destroyed by switches that are things like, "I’m stupid" or "I'm not worthy." The challenge is that these nasty beliefs are buried deeply and generally not directly noticed. You have spent your whole life building an identity or persona to be sure no one ever finds out. I, for example, am automatically a really nice, good person so you don't find out that I’m "no damn good." When that "shame"' gets scratched, however, I am immediately on the attack or frozen depending on the situation.
If you have the courage, start noticing all of the times that you are suddenly upset (can be very mild to violent) and find the pattern. Practice catching your reaction more quickly as you learn to self-observe. Once you can catch the reaction you can correct your behavior to one that you choose. Good hunting.
Photo credit: https://www.flickr.com/photos/33852688@N08/4319647290 Image has been adapted