My theory:
There is a very thin length of string that goes across the whole room. This length of string can be made to go up and down by people outside of the camera frame.
For example, at 2 minutes and 50 seconds in the diabolo is Grinding on top of string that is going up and down very slowly.
Also, at 1 minute 30 the diabolo is on the 'invisible' string making it look like it is levitating.
Lastly, At 2 minutes and 5 seconds in he throws the diabolo up so it hits the bottom of the 'invisible' string which makes it look like it is bouncing back down for no reason.
This theory is not perfect though, I can't explain the vertax other than by saying it involves some knots with the really thin string. Either way the effect is outstanding
