system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how the slightest tap on my balls hurts like crazy, but they feel no pain while flailing around during sex.

Though I'm not sure this would (or more importantly should) ever be explained to a 5 year old...
gpt
Your testis are "hanging out" most of the time; relaxing.

When you do the deed, the muscles controlling them are tensed, providing a level of protection, similar to tensing your muscles before a hit in order to lessen the impact/damage.

Additionally, if you go too hard, you can "feel pain".