Start with a Split Bottom Mount. Bring your right hand up and your left hand down in front.
Do an underpass with your left hand ...
... and bring it back up.
Now you basically do the same thing with your right hand. What looks like an underpass is actually ...
... flipping the yo-yo over your right index finger ...
... into the front string ...
... landing you back in the split bottom mount.
Repeat several times. Every time you do this, the string winds once around your right index finger.