Basically having a flashback throws you back into the moment you were traumatized originally in terms of how you feel emotionally and physically. Your fight or flight reflex is triggered. Your pulse rate increases, you can feel your heart race, you are hyper alert, looking for the source of danger, and sometimes you freeze in place, unable to act. You might physically lash out if you are very disoriented. As time goes on you learn to talk yourself down in your mind. I have found that the flashbacks get less severe in nature and that my heart doesn't race as fast, they don't last as long, and I am better at reorienting myself. But I still have them. I still try to avoid strong triggers like being yelled at or having things against my neck.
In the hospital the nurse had to point out to me that my collar wasn't, in fact, pressing on my throat. I realized that the choking feeling I had was a flashback, not a true thing. It's like that.
For a long time after the Loma Prieta earthquake, many of us jumped if a big semi drove by, shaking the building we were in. We were all ready to dodge falling objects or dive under a table. :)
http://www.mnwelldir.org/docs/mental_health/flashbacks.htm for more info.