Theremin

How the Theremin Works
Vertical antenna: Controls the pitch or frequency; bringing your hand closer raises the pitch.

In this app, this is replaced by horizontal finger movement.
Horizontal antenna (loop-shaped): Controls the volume or amplitude; bringing your hand closer lowers or mutes the sound.

In this app, this is replaced by vertical finger movement.
Electromagnetic field: The player's hands modify the device's radio waves to create notes in the air.
In this app, this is simulated.