You can clearly hear the way he starts with a pure and open 'aa' vowel then from middle C and up to F he rounds out this vowel to an 'oh' or 'ao' (the key being that he rounds the vowel to make the middle darker and apply more head voice) and finally from F# and up he covers the sound which is based on the pure and dark 'oo' vowel. You can hear an immediate change in the sound: from F# it picks up a whole different dimension. This is what happens when you properly 'cover' the high notes. Personally I find the term turning the voice (girare) more instructive because it is a more accurate descriptor of what the singer feels. It feels like shifting into a higher gear to put it in very simple terms.
