In the new number of the Swedish language magazine Språktidningen ("the language magazine") there is an article on the Northern Swedish [shoo], the "yes" that is pronounced while breathing in. It's a popular thing to use when parodizing northerners, for example saying that Northerners are so silent and economize their words to such a degree that they even use their inhalations in speech, but the article explains that people all over Sweden, Scandinavia and the World speak while breathing in. It's called "ingressive speech" and there's even a website about it, made by a Swedish linguist, ingressivespeech.info.
The article explains that this feature is especially common in Scandinavia (but [shoo] is only found in the North), and they mention a sound clip on the aforementioned site with a Faroese person pronouncing the whole phrase "I don't know" while breathing in. This felt very familiar, I do the exact same thing! And, being from the north, I have always used [shoo] (although mostly when I am in that environment), and it should really be written .jo.
Further, the article explains that .jo is really a variant of ja (Swedish "yes"), since Northerners use jo (equivalent of French "si", a "yes" reply to a negative question -- I find languages that lack that word really odd!) when they really mean ja. I had no idea I was doing this until a person from Stockholm remarked that I used the wrong word! But in the end, it's normal, it's dialect. And thus sacred, of course; dialects are like religions.
ja = yes, reply to a positive question ("Do you like soup?")
jo = yes, reply to a negative question ("Don't you like soup?")
.jo = yes, reply to a positive question, and a little bit more...
However, it's not really that simple. You can't use .jo in all places where you could use ja, since it simply isn't powerful enough of a word. It's a word (or sound) that is used to show that you are listening, that you agree, and that you don't want to continue the conversation. It's a closing word (although I'm not sure I've ever used it as such). The article points out that it could be dangerous to answer .jo to "Do you love me?". It would, in my opinion, feel rather like you answered "yeah, I guess I do".
Whereas people in other places in Sweden don't use .jo, they still breathe in while speaking, notably when saying yes, thus .ja. I do remember listening to an Australian comedian who had some shows in Oslo last years and who found this hilarious (both for yes and no I think), so that proves that Norwegians sure do use it as well!