Here’s an idea: What if when you have a feeling in your body, sometimes it’s there for others to see? What if feelings use the body as a display?

That framing sounds dualistic and as if feelings are somehow separate from the body, so that they could be displayed by the body.

Evolutionary the main job of brains is to coordinate movement. Trauma responses like fight, flight and freeze express themselves in movement not because it's beneficial to display them in the body but because movement is required to effectively respond. 

If you truly want to understand what goes on, it's also important to be conscious of tension being possible to by held by fascia in addition to being held directly by muscle tension. 

What if feelings use the body as a display?

I can attest to this theory.

On the days in which I overindulge in my favorite vice (the internet), I carry around a sense of shame. When feeling shame (typically located in my chest), I notice that my chest caves inward (from trying to hide my inner anxiety) and I slouch. Perhaps it's my body telling other people that I'm not feeling confident in myself and that they should avoid talking to me.

Whereas on the days in which I don't use the internet at all, because the shame is not present, I have good posture and can effortlessly chat with anyone I encounter.

Being tense might also be a direct threat to your health in certain situations. I saw an interview on TV with an expert on hostage situations some 7-10 years ago and he claimed that the number one priority for a captive should be to somehow find a way to relax their body. He said if they are not able to do that, the chance is very high that they will develop PTSD.

There is an oft-repeated hypothesis, which I partially agree with, that it also works in reverse, and possibly in a feedback-loop pattern:

• feelings cause muscle tension ->
• muscle tension causes minor social misalignment, which leads to more negative feelings
• tension tricks your body into assuming the situation is stressful, even when it is not
• prolonged tension causes physical pain, discomfort, and reduced mobility which greatly contribute to stress and reduces overall happiness and confidence 

• releasing the tension not only prevents body injury, but improves the mental state (if anything else, because muscle relaxation releases endorphins, and reduces cortisol).

  All of the above seems like common sense, and is assumed and promoted by various masseuses, physical therapists, yoga instructors etc, but Im not entirely sold on it, as I haven't seen any scientific research that confirmed it conclusively.
  
  For what it's worth, my personal experimentation in this is mostly inconclusive, but weakly points out towards the possibility that reducing muscle tension makes one happier and more attractive:

  - for a few hours after a muscle untensing massage, hot bath or hot sauna, I'm not just far less physically tense, but also significantly more emotionally relaxed and pleasant to be around, which might not make me more attractive, but possibly prevents me from acting in an unattractive manner

  - OTOH, I also feel greatly emotionally relaxed after a thorough weight-lifting session or an intense run, despite these things causing severe muscle tension, often to the point of pain and cramps. (IMHO, this suggest that the endorphin release from using your muscles is an effective mood changer, and thus personality changer regardless of lingering muscle tension)

  - im definitely more social and effectively attractive while mildly drunk (this has been extensively tested for decades). Alcohol is a minor muscle relaxant, and my drunk self is definitely more physically fluid, but whether this makes a measurable difference on attractiveness is hard to tell, since alcohol is a major dis-inhibitor which has vastly greater effect on producing bold, attractive behavior.

  - confusingly, I noticed that both acting visibly relaxed AND acting visibly tense (or rather, intense) seemed to attract women in different context. My assumption is that being visibly physically relaxed is a sign of emotional confidence and strength, but visible tension can be a sign of decisiveness, aggression and focus; which can be interpreted as dangerous or sexy, or possibly both.

  - relaxed muscles contribute to a confident body language, which I think contributes to attraction, but not always and not in every context or with every woman. From personal observation: it seems that there is a complex interplay between how women perceive man's body language, how they perceive him socially, and how they perceive the environment where they interact with him. I evolved through different stages of physical fitness over time, and they played differently with body language. Relaxed, confident, untensed body language that causes the man to physically open, "spread" and sprawl over the environment seemed to enhance my attractiveness when I was physically buff, but was perceived as annoying and arrogant when I was overweight. It was almost as if I was acting "above my station" when showing relaxed confidence in an unattractive body. Similarly, the untensed body language worked far better in casual environs where it was expected for a man to act that way (a bar, a club, a house party etc), but seemed to have the opposite effect during "daytime" interactions like a lunch-date.
  
  So my tentative hypothesis is that untensing your muscles has a compounding effect on a man's perceived attractiveness, positive or negative depending on context.
  
  Im not sure if this has similar effect when women physically relax in the presence of a man. I notice I find physically tense and relaxed women equally attractive, though possibly in a different way.
  
  It would be also interesting to research this in the context of same-sex attraction. As far as I can tell from observing my friends, gay men I know tend to be more physically relaxed than straight men in general, but their body language hardly seems to affect their attraction to each other. Inversely, the few gay women I observed in social environments, tend to be very physically tense and visibly anxious in the presence of other gay women, to the point I had a hard time at first to tell if their interaction was flirting or passive hostility.

Yes, and:

Muscle tension probably also is a direct result of stress. Keeping muscles tensed makes you ready to move. Being ready to move is necessary in most evolutionary cases of stress.

Muscle tension isn't a zero or one; muscle control signals are on a continuum. So the question is how much muscle tension and where; we always have muscle tension, even when asleep.

I do find the jamming hypothesis credible for some types of tension and situations, but it's not the whole story.

The question of why some areas of the body develop more tension is an open one, and jamming signals is probably one cause. But there are other possible feedback loops that could connect some body parts to stress/tension.