Interestingly, some studies have done of people "speaking in tongues". One interesting result is that when they do that they tend to use a selection of the same phonemes as they use in their regular speech/language. This tends to suggest that they are speaking a kind of nonsense mish-mash drawn from their usual language, rather than some kind of strange foreign language.
It tends to happen more in social environments where such behaviour is expected and socially acceptable.
In some ways, I think the effect can be a bit like certain kinds of meditation, like reciting "Om" over and over again. It can leave the person with a feeling of clarity and even euphoria. Doing it in a church setting, surrounded by friends, can be a bit like dancing furiously in the mosh pit at a concert.
