Google "Present tense in Spanish" or "Present vs. Present Progressive in Spanish" and you will see a number of good resources that confirm that there is considerable overlap between the two senses. So yes, yo descanso can be translated either "I rest," "I am resting," "I am about to rest,"--and none of these choices are wrong in the real world!
I suspect that what is happening here is that the Lords of Spanish Duolingo just want us to avoid confusion in the beginning by positing a clear, though artificial distinction between the present and present progressive tenses. Thus, the 'bots will only accept "I drink" as the official translation for "yo bebo" and ding us mercilessly for having the temerity of writing "I am drinking." This happens with other languages Duo covers as well, tho it can be real annoying for those of us using the language for so many years. Unfortunately, like so many others, all I can advise is report "My answer should be accepted" each time the issue shows up and hope that one of the designers eventually takes notice!
Why is "I rest my ass" wrong ???
Why is "Just putting my feet up" wrong ???
Why is "Taking a time out" wrong ???
Why is "Just goofing off a while" wrong. ???
Might it be that the only thing that is important here is understanding what the Spanish sentence MEANS and Duolingo's English sentence provides us with a good enough clue sbout that?
Could that not be the case?
Why stop there? Why not be also concerned about why Duo also does not accepted: Break Interjection: At ease! , Stand at ease.
Duo provides a simple translation to enable to grasp meaning. It is not teaching translation and is not concerned about all the different ways something can be translated into English. And this is THE answer to your wonderment.
This is a stretch for me as a native English speaker. I think the relationship might be that if one thing is supporting the weight of another thing, then whatever is having its weight supported is "hingeing" on whatever is supporting it. BTW, I looked up "hinge" to find out how to spell it when you make it a present participle, and I found this definition: hinge = depend entirely on. I agree that this definition of "rest" is not generally used in the English-speaking world, but it may very well be another way that the word "descanso" is used in the Spanish-speaking world.
Ah now I get it. "The case hinged on the forensic evidence..." would be the same as "The case rested on..." in English but it's a tad obscure and verges on being a metaphor. Presumably the Spanish could use descansar as well as depende de or girar sobre but http://www.spanishdict.com/translate/to%20hinge doesn't show it...
I don't see any great difference between them. You likely wouldn't hear anyone say "I rest," but rather "I have a rest" or "I take a rest" (assuming that you do this on a regular basis. For example "After lunch, I take (or have) a rest."
If someone disturbs you, you might say, "Quiet! I'm resting!" (or "taking/having a rest) (I'm resting right now) but I can't think of a situation in real English where a person would say "I rest." It's not incorrect but it sounds strange to my ears.
"I have a rest" is not a usual English sentence because in this context the word "rest" is usually used as a verb rather than as a noun. The noun usually used is "nap" if the rest is a short period during the day or the infinitive "to sleep," as in "I go (am going) to sleep." In this sentence the infinitive "to sleep" is being used as a noun.