When the copper strings come into contact with the skin a circuit is closed which in turn generates sound. The small changes of the skin's conductibility - caused by reactions to outer as well as inner stimuli - are what modifies the emerging sounds in pitch and volume.
More specifically, the tones are modified by
varying the pressure of the bow on the skin
playing on different areas of the body's skin
the changing of conductibility caused by perspiration (nervousness etc.)
sensors attached to the right hand which close further circuits and therefore allow for additional musical effects