I was wondering if anybody knew what kind of preshortened string comes in the box of the loop 720s (jap version). I’m pretty sure they’re 100% poly but I don’t know where to buy more of them from.
They’re perfectly responsive for my 720s, even after thinning out a bit. The length of them is also perfect for my height. The first time I tried them, my consistency went up exponentially.
I’m like 80% they’re poly strings that yyf makes, they have for sale in bulk on their website.
I think @gcoomans is right. It makes sense that yyf would stock there yoyos with there own string. Just a side note the standard length on yyf string is really long for looping. I would recommend cutting your string to a few inches below the waist.
Thanks for the suggestion 
No prob! I loved the string that came with mine, but I recently tried some type 6 white poly, and I think I like that stuff better, you should give it a try!
2 years ago I got good at one-handed looping - 25 or so most of the time but that was only after I shortened the string significantly - to the length you suggested.