Whistle Voice or Whistle Register? What is it?
This texture of sound is produced by an extreme stretch, even to the point of the larynx rising a bit. It is only found in the female voice at the very tip top, above high C or D and extending to around the A about a fifth up.
One may think of Mariah Carey as an example of who can produce these sounds in the popular world. In the classical world, one can get a feel for this sound by listening to the Queen of the Night arias from The Magic Flute by Mozart.
These very top notes, just like the notes before it that are at the top yet not quite so high, begin as very small pinpricks of sound. Yet, if they remain this way the voice becomes constricted.
These notes begin like tiny little dots way back and up in the head as a feel. With slow careful development, the female begins to crescendo and decrescendo on these notes. These notes become a legitimate part of the vocal range, used in a healthy way as a part of the full voice, like standing on the roof of the vocal house.
One is advised against extended periods of this extreme stretch for if all is not carefully balanced with the closer muscle then there is a risk of the cords separating and issues appear, like the middle of the full voice starting to disappear or becoming breathy and weak.
Most women will not need these notes in day to day singing. But, if these notes are there, then it is good to stretch and claim them for the purposes of general vocal health.