Trying out an instrument being developed by composer Hallvarður Ásgeirsson. His aim was to excite the strings of the guitar by the use of transducer attached to the body of the guitar and picking up the vibration of the string with a pickup.
What happened was even more exciting, the feedback between the pickups and the transducer was mainly connected through the body of the instrument itself, resonance frequency of the feedback arrived from modes of movements in the body. By damping vibrating modes, it changed the frequency of the feedback.
See further on Modal Analysis
Video by Hallvarður Ásgeirsson: http://www.hallvardurasgeirsson.com/