What is the player's role in the functioning of a theremin? I recently see a video on how the theremin works, and wasn't satisfied with the answer. I watched around, but they all seem to give the same explanation.
A diagram as below is given, and it is claimed that the theremin contains one plate of a capacitor, whilst the player's hand makes up the other. The diagram itself seems to dispute this, for whilst diagrammatically, both plates are connected to the circuit, from what I can tell, the player is in no way connected.
How does this work?

 A: The player's hand acts as a grounded plate remembering that the player is a reasonable electrical conductor.  The capacitor is part of an inductor-capacitor circuit, as you have shown above, which control the frequency of an oscillator.  So what is missing is a clear indication that the bottom part of the circuit is connected to the earth/ground.
A: In your layout, imagine the "antenna/you"-capacitor being parallel to the existing one, the antenna being the upper plate. Parallel capacitors add up their capacity.

So how do you become the plate although you are not connected to the wires? 
The first step is to understand is that this setup (inductor + capacitor) will generate frequencies, as you could see in the video. The frequency is determined by the properties of the inductor and the capacitor (so if you change it by the antenna, the frequency changes). If you put a speaker parallel to the capacitor you could hear something.
Assuming no resistance, the oscillation would go on forever, you would not need to connect it to a power source after an initial load.
You can add a dc power source  to make up for the losses caused by e.g. resistance or the speaker. The "-" in a circuit is just "GND", it's basically a connection to earth (it sets the absolute potential that we call 0 Volts). Keep in mind: the oscillation will still go on.
And now to answer your question: You are the earth to that small antenna-you-capacitor, you can think of it as if the earth closes the circuit. (It will still work if you jump or stand on an isolator, since you are large enough.)
If you would plug this directly into your home, the power company might send you angry letters because you send your oscillations backwards through the powersource, and you don't want the full voltage on the antenna neither (in case you decide to touch it), and you probably want nicer sounds, so in reality, the circuit looks a little more complicated. But in principle it would work.
