I think every French teacher who taught me in elementary and high school was an over the top expressive human being. As a result, I have a very clear image of exactly how narrow they made their cheeks, and how far out their lips were when they were demonstrating ‘eu’ – as in ‘petit peu’ (a little bit)
If you’re not sure about what this mouth shape looks and feels like, imagine that you need to make your singing heard in a large area on the other side of a fence made of tall wooden boards. Now imagine that there’s long sliver of one of these tall wooden boards that’s missing – so there’s a vertical break in the fence that’s about one inch wide. If you were to make your cheeks go skinny, and stick your lips through this split in the fence, that’s what this mouth position would feel like.
This will feel ridiculous.
As crazy as it seems, when people’s mouths are in this position it’s much easier to synchronize fast moving lyrics, and the vocal production moves forward in the mouth. It’s also a useful mouth shape for blending and smoothing out the sound. Probably not what you want for anything loud or brassy.
One other important benefit of practising this mouth shape is that it gets singers used to singing through a very narrow mouth shape. The lips can be more relaxed eventually, but singing through a consistently narrow column will eliminate any need for ‘switching registers’. No more necessity for ‘head voice’ and ‘chest voice’ – just one smooth production technique from the bottom to the top of the range. Singers can still call on their ‘Sky Voice’ or their ‘Ethel Merman’ voice, but only for colour purposes. Practising the join from one voice to the other is no longer necessary.