I recall doing some practice with a butterfly knife back in the day, (balisong), I'm assuming you know how they flick and twirl them to open and closed positions. Anyway, many cuts later I mastered it. My point...I wish I had a blunt one to practice with. Lol.

drew it across his thumb...and cut it to the bone

Deserved it. Although, that's pretty harsh I guess, I should remember that people are so clueless.