system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes people to 'jump' when frightened?
gpt
It’s An instinct that causes your muscles to start tensing up as if you’re about to have to quickly move or fight something.

There’s a reason that people when startled often stand up and/or being their arms up closer to their chest/head, it gives the body and head some extra protection than if your arms were at your sides and when your arms are in your  pretty much ready to start punching.