This is the original Wired Hot Hand Sensor from Source Audio. After plugging its 3.5mm connection into the sensor input of, say, a Soundblox pedal, simply wrap its ring-like accelerometer around your finger, and you'll be able to modulate parameters on your stompbox by moving your hand around.
With this sensor, any effect modulation that utilizes expression control, which would typically be regulated using your foot—wah wah or evelope filtering, for example—can be achieved simply by waving your picking hand. Also, many Source Audio stompboxes allow you to assign various parameters to Hot Hand control, so you can achieve esoteric manners of manipulation not available to conventional expression pedals.
With Hot Hand control, facilitated by this ring pack, you're free to give your foot a break, or to use your shoe to modulate a different effect. A clip is mounted to the cable to attach to your shirt, making cable management less of a chore.