I know I would give my life for someone else. It is just that I have no idea what the scenario would be.
I will explain by giving an example:
A little child dashes into oncoming traffic in front of an oncoming truck.
Without giving it any thought, or because of my "instincts", or on an impulse, I rush into the street, shove the child away from the oncoming truck and get runover by the vehicle myself.