The fight/flight response (or what I have referred to as the 5F response) is as follows:
Examples of single instances of Initial Sensitising Events (ISEs)
Take a soldier in combat who is under constant heavy gunfire and "hell is raining down on everyone around them". Fellow soldiers in their troops are killed in the process, and maybe they have come close themselves. Maybe they have lost a limb from a mortar bomb explosion. This can be one ISE which on its own causes PTSD.
Take a man/woman who is threatened with their life and raped at knife/gunpoint. Again, this ISE alone can cause PTSD.
The fight and flight mechanism
A more in depth rundown on this (what I have called the 5F response — fright/flight/fight/freeze/fawn) can be found in my answer to Understanding fear as a response in classical conditioning, but in short, the 5F response is an unconscious reflex to perceived threats (loud bang or other stimulus received at an ISE including smells). The process follows the pattern Perceived Threat → Brain Receiving Signals → Brain Reacts (Fright) → Cortisol and Adrenaline is released → Physical Reactions occur from the release of hormones → Bodily response (Fight, Flight, Freeze or Fawn)
From experiences provided by those I have helped who have suffered trauma, there is a 6th possible response (flop — fainting or just plain collapse with fear) making the 5F response, the 6F response (fright/flight/fight/freeze/fawn/flop).
Adrenaline and Cortisol Release
As you can see within the infographic above, the brain initially processes the threat signal in the amygdala and then the hypothalamus and the pituitary gland secretes adrenocortictropic hormone (ACTH) which leads to cortisol and adrenaline release to enable a speedy escape if chosen.
Cortisol is constantly released to maintain the stress response. Once the threat levels lower (either the threat has gone or you have later realised there was no threat after all), the amygdala and hypothalamus are no longer stimulated by the threat and so cortisol release is stopped.
As the cortisol is spent in the body, you start to reach your normal state of calm.