Why does Ayu move her mouth to vibrato?

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well, I heard that her way of singing is like traditional Japanese singer,, that always end every line with high pitch and vibration,,
but,, about that mouth, I don't really know.. Maybe it's a habit like Namie always pull the mic down when she wants to sing in high pitch..
I don't really like it,, and I often made jokes about it,,, XD
she should use her old fashioned way of singing...