In the motion of Yuuki slack where you pinch and slack into a double or nothing, I can’t find a good way to pinch the string so that I can rotate the slack around twice before doing a double or nothing like in this: http://instagram.com/p/zNQaAfxAR0
Anyone know a good way to pinch it?
…between… your forefinger and thumb…?
I’m confused by the question. 
The string that the yoyo is sitting on gets all twisted, and if you swing it around twice its hard to keep pinching it
Yup, it gets twisted, then untwisted if you go back the other way or insert a finger. It’s weird to hold onto and not like any other trick I can think of… but forefinger and thumb gets the job done.
Yuuki Slack is definitely a learn in person trick. Its much easier to learn it when someone is doing it in front of you rather than a video. Just search around for tutorials that really break it down or give you a slow mo version. It’ll come to you.
Don’t unpinch your fingers when you do the slack. It will get twisted up, but then it should untwist when you go in the opposite direction. Just like GregP said.
Im talking about the first slack where you go from trapeze to double or nothing, not the actual Yuuki Slack
Once you pinch, you don’t let go. So the answer is the same: forefinger and thumb.