Electromagnetic 'aerial'

OK new to the forum, thanks guys, it's excellent. Will spend many a sunday afternoon on here!

Saw Joe on 5th at Blackpool and was intrigued by the aerial that Joe used to get sounds out of by holding his finger close to it. From my knowledge of physics and electromagnetics it seems he was  generating some sort of electromagnetic field/circuit between the one antenna that pointed at the guitar pickup and the other antenna that his finger pointed to, with Joe himself being part of the circuit . So what happens is a circuit gets formed between joe's left hand on the guitar strings and the finger nearing the antenna - thus creating a sound from the pick up on the guitar that antenna #1 is pointing to. (my 'theory/idea' on what was going on!)

Does anyone know any more about what this thing is and how it works?

All the best