The arpeggio is an effort to help shift registration and vowel modification higher in the voice. The singer ascends on a pure [a]. At the top note, a closed vowel ([i] or [u]) is introduced. It is slightly changed to the next closest closed vowel ([e] or [o]) and is then returned to [a], on which the singer descends.
The challenge is to leave the voice “gathered”. This is the term my teacher uses to describe the clear focused feeling that accompanies good phonation. You can easily hear the absence of this in young singers when they try to sing open vowels; the tone is often breathy and spread. Here, the vowel begins closed and “gathered”, and as it is slowly opened towards [a] (without pausing the phonation) the singer’s tone must remain in that “gathered” state.
This can really add to the clarity of tone and ease in high notes. It’s also a smooth way to introduce vowel modification.
If you have any thoughts, let me know.