I think that people don’t take the time to get used to shorter string when they try it. I cut it a little below my belly button, and it does wonders for my speed and accuracy. Then again, I’m very tall, so your results may vary 
^^^This. I’m about 6’2’’ and the slipknot stopped about 2 inches below my belly button with the yoyo resting on the floor.
bellybutton
my way to figure string length:
The length of the string matters. Everyone always says “preference, preference preference” which is true… but to every “sport” there are some rules from pros or whoever that most people realize work best. So… Even if you think its your preference I would experiment around if you havent already. I would say the variables that go into the length of the string are first the length of your arms/wrist/hands then your height/throw (so it doesnt hit the floor). Your belly button can be proportional to this but it probably isnt exact. I cut it from my belly button but ive noticed lately its too short for slack tricks even some BINDS i would like to do.
Don’t blame Andre for saying it in the video. I’m sure that works for many people and especially at the time, was the trend. It seems his YYE string works for most! I tried another websites pre tied string that was different colors and it was too short. I think YYE is too short for me but thats because I am pretty tall…and probably used to cutting from my bellybutton. I was addicted to Chaos string. Don’t think its around anymore.
I recently started cutting it longer and I noticed it has helped…and I have been yoyoing for 2 years. It REALLY helped on slack tricks. I also noticed that I hit the floor a couple times so I made sure I used a “cheaper” less rare yoyo to get used to it. Also over carpet. I eventually got used to it and dont hit it anymore. Just better slacks and definitely whips. Kwijibo is cleaner.
I think the best way to tell if the string is too short/long to your preference:
if you learned and know it well: Do a kwijibo. On the second pop if it lands around your NTH wrist instead of your finger (just like the beginning of yuuki slack)…or on your finger/hand too close to your body… its probably too short.
If your just missing and it feels difficult to land (best way I can explain)… maybe have to pull your hand really far apart and pop it higher… inconsistent… its too long.
If you can land it. ;D Your good! (just my experience)
tip: also make sure if your cutting string (not pre knotted), make sure you are consistently wearing shoes/not wearing shoes when you do it or it will make some difference. Also if your doing it from your bellybutton make sure your standing up straight (or the same posture every time).