Singers that bare their teeth

Hi there... Was wondering...Why do so many great, but ‘untrained’ singers bare their top teeth so much when singing? I see their lip go right up and their upper teeth come out all the time… is this a tension habit to try and avoid?

Or maybe is it a kind of semi-spread belting manoeuvre?

It could be both/either! Good observation! You're correct that spreading/widening the mouth is a belting technique (which could be employed by trained and untrained singers alike). Spreading sometimes goes hand in hand with shouting, which is to be avoided. But it's hard to determine whether something constitutes "excess tension" just by the look. For your own practice, feel free to experiment with widening the mouth into more of a "megaphone" shape, using more "spread" vowels. Pay attention to the sound and the sensation, and adjust as needed. Return to a more "neutral" up and down "UH" shape if the sound is getting shouty/uncontrolled, or if you're feeling excess heaviness/tension. 

That's a great tip, thanks Camille. I was watching (super trained) singer Morgan James the other day (she seems to have taken lessons her whole life - https://youtu.be/lGaz3BG_iSk?feature=shared ) and she does it all the time even when not belting so maybe she is able to employ it without any tension 

Yes, she looks and sounds very in control of her voice! I think anatomy plays a big role as well. Some folks will just show more teeth because of their facial structure. (I'm a huge Jeff Buckley fan, so thanks for sharing the link to that cover!)