I know this effect is real but I also know, as you said, that not all people respond the same way.
If you will forgive a personal account here is another reaction. Once I was confronted with a situation in which there was a very good chance I would not survive. The details are not important except to point out that it did not catch me unexpectly. I had a few hours to think about it. When the event did happen, I was surprised at my physical and mental reactions.
First of all my mind became crystal clear. I had a feeling of invincibility. I am sure the feeling was to an extent false, yet I also think I really would have been able to handle any situation better than at any other time in my life. I cannot stress enough how confident I felt. I absolutely knew I could deal with anything. Fortunately I did not have to test my new found powers.
I have never read anywhere of that effect yet am reasonably certain most others would react the same way in similar circumstances.
I would be interested if any others have experienced the same.
That describes my feelings several times when under fire and returning fire. There is a clarity to combat that can come upon one. I believe it may be due to the adreneline pumping through one's veins combined with a reliance on thought over a merely physical reaction. That said, the actions one takes during a combat encounter are based more upon something other than merely training. The training helps and it really can weed out many who would not be able to handle the situation but training alone is not equivanent to having seen the elephant.
Stay well - Stay safe - Stay armed - Yorktown