[2007] Ayu's vibrato thing

[Neo Daydream]

Ok, Ayu's vocal style and skills have progressed and evolved over the years.
As someone who has studied voice and is a singer herself, I think I've come to understand the aspects of Ayu's voice. But one thing puzzles me...
sometimes Ayu will be singing her little heart out and at the end of a long note her jaw moves back and forth and she creates this random extreme vibrato. Do you guys know what I mean?
if you do, do you like it or no?
what are your thoughts on how and why she does it?