I started throwing my first curveball when I was 17. I never had a coach who really showed me how to throw one so I had to do my own research . it has developed into something pretty nastya. I made a couple of personal adjustments to what I have read about it in the Internet. Normally you let the bottom seem lay on top of your thumb ( at release point) so there is no resistance fro the seem having to flip over the thumb. When I trieddoing this intentionaly though I figured I had way more spin on the ball as the seem seamed to pop out of my thumb and gav it more spin on the ball.
I'd love to be able to explain this a little bit better but I don't have any pictures...
Tell me how you throw/hold your curveball and I'll be glad to tell you a couple of tricks..