Dave Laing made comments on the vocal style of Buddy Holly, in a chapter of the book On Record: Rock, Pop, and the Written Word. It has been difficult for me to find any commentary about embellishment in singing. So I am happy to present what he has said. I add my own commentary.
Contrast in Intonation
I call this using different voices. Laing gives only one example, the use of a "nasal, babyish" voice in two lines of "Peggy Sue."
Laing offers no explanation for the use of this contrast, except to say that it is related to the music, not the words.
Contrast in Pitch
Laing says that Holly borrowed this from Presley. The "contrast in pitch" is putting high notes and low notes close together within a phrase, to create contrast.
However. The song "That'll be the Day" has the phrase "That some day well I'll be blue." "I'll be blue" is sung on lower notes. That would not be remarkable, in my opinion. What is interesting is that Holly also changes the voicing, to a large throat (producing a deeper sounding note). This is especially exaggerated on "be blue."
Again, Laing lists no musical reason to use this effect, except as a interesting embellishment.
Laing does not define what he means by this. One example is breaking up a long note with "hiccups". So Laing is probably talking about effects that could add rhythmicity (discussed href=rhythm.htm>here).
Laing suggests that these hiccups function to break up the natural sentimentalitiy of a ballad, and to attract attention. Of course, they would break up sentimentality, if that is an issue. And I think Holly did do some things (like the nasal voice listed above) just for the sake of experimenting and being different.
But I suspect Holly was adding rhythm. As for Presley, the instrumentation and drum are not rocking in Holly's songs, so it was left to Holly to put the rock in his songs. I think he does. It is difficult for me to analyze where Holly puts the extra rhythm, but given that he succeeds in adding rhythm, he is probably doing it with his "phrasing".
Laing does not mention it, but Holly uses "falsetto breaks", which are brief flashes of falsetto at the start of a note. There are at least two in the song "That'll be the Day".