system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why we instinctively clench our teeth/jaws when exerting force

Like opening a tough jar, hitting something, or just anything that involves force.
gpt
It's historically (in an evolutionary sense) been beneficial to brace those parts of our body when you use force (because presumably this often happened in a fight/struggle and this reaction minimises risks in those scenarios - of e.g. breaking your jaw). So when you enter "combat mode" and you ready to use your muscles, this is your instinctive response. 

In other words, you've evolved to do this because it was more beneficial than other potential instinctive reactions in such situations in our deep past. 

Good guy natural instincts got our back, even if we just go into "combat mode" to open a jar!