This is even more fun with two people playing! One person controls left/right, the other controls jump.
Its not just limited to the theremin. Its even possible to hook up a microphone and use your voice to control the game! Or a guitar! Or a violin!
Heres how it works: The sound from the theremin is split into its frequency and amplitude components in real time, which are then mapped to values in a linear scale representing the X and Y axis. Pitch becomes horizontal control, and Volume becomes vertical control. The X and Y scales are then cut up into different zones. In this case, Left; Right and dead zones for the horizontal, and a single trigger and dead zone for the vertical.
The trigger zones are then mapped onto a virtual joystick hooked into an emulator.
The end result is a fairly usable input control for playing games like mario. The bars give the much needed visual feedback as to how "in tune" you are, so you have a better feel of where the trigger points are.