The Thumb Cast On is found on page 67 of RD KH.
You can find a video for this cast on on KnittingHelp.com, titled “Alternative Cast On.”
This is a long tail cast on worked with two strands of yarn and one needle. It isn’t exceptionally stretchy, but it isn’t stiff either. It looked better when I treated it as a CO + first knit row, so I did a purl row as my first row. I found it a little fiddly and slow to work. I’m not sure if there is any structural difference between this cast on and my normal long tail cast on.