The Thalamus region of the brain gets sensory data. Parts of this is sent to Amygdala(emotional/irrational part of the brain) and parts to the neocortex(thinking/rational brain). If the Amygdala matches it with an earlier threat stimulus, then it activates a shortcut(HPA/hypothalmic-pituitary-adrenal axis) - and effectively takes over the brain overriding the rational area.
Emotional brain works faster(by milliseconds) than the rational parts - so Amygdala can act before instructions from neocortex as ready. This is to improve the surviability in threatening situations. Some characteristics of such a response include...
- strong emotional reaction
- sudden onset
- post-episode realization if the reaction was inappropriate
The pause method can help with this - but it requires training. You'll have to teach your neocortex to inhibit your Amygdala.