The amygdala is an area of the old brain deep in the limbic system it's responsible for fear and fear responses. It releases a cascade of chemicals in the body stress hormones like adrenaline and cortisol they flood our system immediately preparing us to fight or to flee so when this deeply instinctive function takes over we call it what Daniel Coleman coined as amygdala hijack. We can't choose how we want to react because of those instinctual protective mechanisms in the nervous system but there's a way that our brain kind of prepares us for it.