Could you give some insights to how you double thumb in regards to hand position, what part of the the thumb hits the string on the up and down, etc...

Thanks

I use the tip of my thumbnail for the up and the opposite side for the down. The trick is to use your thumb as a pic and don't dig to deep on either attack...relax and remeber your thumb should stay rigid.
hope this helps..........

That helps a lot. Thanks:)