system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does holding in our breath allow us to exert more force?
gpt
Valsalva (holding your breath) tends to help stabilize your core by increasing intra-abdominal pressure, allowing you to apply maximal force. It is like trying to push something with a wet noodle vs. a dry one. 

Obviously this is only really important on maximum effort things and does raise your blood pressure, so if you have BP problems, it can be dangerous, and there are various dangers to doing this, including passing out from lack of air or blood pressure changes and stroke (more of a danger for old people), so don't be an idiot.