system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening physically to cause the clenching, sinking, contraction in your chest during a jump scare

Wife just asked this exact question while almost being merged into driving down GA 400. Trying to bring her the joy of Reddit this holiday!
gpt
It's your body reacting to the sudden release of hormones like adrenaline. It causes may reactions in your body, making it ready for fight or flight. Eyes dilate for better vision, heart rate increases to get blood pumping and your muscles tense to convert your body into a loaded spring ready for action. We all now these sensations, just like you described. 

The next question could be: how is the body able to release hormones so suddenly? These are processes in the "older" parts of the brain, older as in "evolutionary older": the brain stem and limbic system. Processes in these parts bypass cognitive neocortical thinking and can recognize dangerous situations pretty well. This process is also totally involuntary, you can't suppress it. This process can also be fooled, for instance by an exciting movie or a fast moving object you notice in the corner of your eye that is actually just a fly or something else that is harmless.