Snippets and Links
This webpage describes three different "embellishments made by singers in the early days of rock and roll. Staccato is "the choppy diction of rock 'n' roll singers as they match their words to the metronomic dimension of the song's beat." Swivel is "the small-range inflections often occurring in between staccato syllables." Finally, glide is "the high-rising or deep-diving twists to which vowels or syllables are submitted."
A nice thing about this webpage is that these are illustrated with several songs, such as Eddie Cochran's "Summertime Blues."
According to the author, the purpose of these is to "drag the lyrics into unintelligibility." Does this seem likely to you? It is not clear to me that singers want their lyrics to be unintelligible, and the usual understanding is that listeners could ignore intelligible lyrics if they wanted to. Finally, there are many more efficient ways of making lyrics unintelligible. To be fair, the article is moreso about the meaning of the lyrics, not the purely musical role of different embellishments.
To give a more reasonable explanation of at least staccato, it probably helps make the song rock.
This webpage also lists interpretational effects for the guitar. Some of these are standard -- vibrato and slurring one note into another. A bend is to strike the note and bend up. A pre-bend is to bend the note up and then strike it.
Go Cats Go: Rockabilly Music and Its Makers
Author Craig Morrison describes the vocal style of Rockabilly as "full of passionate emotion and eccentricies: raspiness, exaggerated enunciation, added and deleted words and syllables, hiccuping, melisma, feathering and falsetto, interjections, and melodic distortions." He also quotes Steve Tucker's description: "extravagant, even histrionic, vocal acrobatics -- grunting, groaning, moaning, mewling, whoping, whispering, stammering and stuttering."
Apparently, the real start of rockabilly was with Elvis Presley, who uses a relatively large collection of different embellishments (though not all the ones on this list). If rockabilly came first, I could imagine Presley using this wide range of options to good effect. Instead, he apparently helped create it.
Again from the book: "Feathering is the name given to the falsetto grace notes often found at the ends of lines in folk and hillbilly songs." From a definition by Judith McCulloh: "A sudden or forceful raising of the soft palate against the back wall of the throat and/or a sudden closing of the glottis at the very end of a given note, generally accomplaned by a rise in pitch." Presley of course uses falsetto grace notes at the start of notes, which does not fit either definition. The term "feathering" does not seem to have caught on.
tempo-giusto is a strict-tempo style and parlando-rubato is a free, speaking-rhythm style
"The style of traditional singers may include how they release their words; how they approach their pitch (do they slide nasally up to it?); whether their voices are tense or relaxed; whether their timing is metrical, semi-metrical, or nonmetrical; whether the mouth is wide open or partly closed, and how this affects the formation of vowel sounds. Style includes the way some singers ornament the melody with devices such as vibrato, scooping (slurring up to a note), sliding down to a note, and feathering (adding a hook to a note by use of a glottal stop)."