There’s no better proof than hearing it for yourself. These recordings from a small sampling of my Level 3 clients audibly demonstrates the vocal freedom The Shetler Technique can produce.
Broadway: Ain’t Misbehavin, Dreamgirls, Once on this Island
He studied with me throughout his Broadway career, and I ended up producing a club act for him as well as a four song demo to pitch a record deal.
Both of these songs show that when the voice is free it is malleable to any style of singing.