I ran across this term in Thomas (2007). It would be an interesting page or paragraph. He defines it as "the tendency of vowels to show a more schwa-like quality, more coarticulation with neighbor- ing segments, or, for diphthongs, truncation."
"In Thomas (2002a:179–81), it exhibits interspeaker variation in its magnitude and could easily act as a sociolinguistic variable."
"Moreton and Thomas (in press) show how a dialectal variation in undershoot was one factor that led to the development of “Canadian raising” of /ai/, in which /ai/ nuclei are higher before voiceless consonants than before voiced consonants, in the Cleveland, Ohio, area."
Thomas (2007). Sociophonetics. In Bayley, Robert & Ceil Lucas (eds.) (2007). Sociolinguistic Variation: Theories, Methods, and Analysis. Cambridge University Press: Cambridge.