Not really sure how to explain it but the basic idea is that the reaction is harder, let’s say for example there’s an explosion or other loud and sudden sound, if you are distracted you simply get a big stimulus from nowhere and it can probably be dangerous so your fight or flee mechanisms kick in, your pupils dilate, your muscles tense, your heart beats faster, essentially all your senses are enhanced and that makes it feel more intense while when you are expecting it there’s no adrenaline rush so it feels “normal”