This trick is called the Brain Twister and is one of the first string tricks that you should learn. You start off with a throw down, and you've got to have a really good, strong sleeper, because this trick is kind of a long trick. With your left finger you push the string in front of you, come down with your right hand, and you want to mount the yo-yo into the string, like so.
You come up around with your right, and right below your left hand, you want to pull on the string. When you pull on the string you want to flip it around in a circle, like so. You want to try to do as many rotations as you can. Like so.
When you dismount it, basically you hop the yo-yo out over your left finger, like that. There's also different variations that you can do with the Brain Twister too. Change directions. Actually, dismount, come right back in. Over correction.
There's also quick mount to the Brain Twister. Instead of going the back way, it's a lot slower. You just go right over your left index finger like that. By doing it this way you get into the trick a lot quicker.
And that's the Brain Twister.