Double-thumbing is where I use my thumb for up and downstrokes, similar to using a guitar pick": Bass legends Victor Wooten and Stanley Clarke in conversation | Guitar World
Perfecting the Double Thumb Technique – No Treble
Demystifying the Secret to Double Thumbing for Slap Bass Players - YouTube
Thumb Position Technique Lessons — Discover Double Bass
Pro & Contra Double Thumbing - Bass-Workshop - Bonedo
4 Double Thumbing Mistakes To Avoid (Slap Bass) - YouTube