How does he do this?

Yeah, if you look closely you can see it moving slightly.

Easy way is to just get a volume/expression pedal, you can do the same thing.

Well.....sorta.....The effect with a pedal is similar but using the 'pot' is changing the impedance of the guitars output as well as its voltage. Theres a subtle difference between this and simply choking the signal out with another pot in the pedal. The pedal is not affecting the pickups in any way whereas the volume pot on the guitar is. neither is 'better' as far as the violin effect ......
 
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