I was looking for something on my closet and I found some fishing things, amongst them were this small things that are called swivels. They are used on fishing lines so the they can untwist.
Here is how they look:
I cutted the string neer the loop for the finger and inserted there the swivel. After that that I tried looping with my Shinwoo Loop ¨720¨ it looped perfectly and the string didn´t unwind till the yoyo would have come out off the string lieke the string without the swivel would have done. It works really well, in total I did about 300 loops in many throws without adjusting the tension and no yoyo went flying away.
I got the swivel in a fishing kit at walmart but if you want to get a swivel by separate at a store specialized in fishing you can probably get a pair for less than a dollar.
This is how it looks on a string: