Singing vertically by way of narrowing the vowel.

Assistance from the outside, like we do here for Stevie Wonder, and Frank Sinatra does for Sammy Davis Jr,. by holding the face and in that preventing the vowels from going wide, helps the larynx to stay down and allows for vertical singing which enables mixing. Even when the singer already sings in mix like here, vowels that are too wide are still taxing for the voice.