Edelrid Zap-O-Mat:

This is marketed as the world's first adjustable belay device. The idea of having a loaded spring in the handle means the Zap-O-Mat is ideal for every sport climbing situation and for every belayer regardless of weight or rope thickness. Soft or dynamic belaying is possible without having to jump. Push the green "spring load“ button in (as in the picture above), and the rope can be paid out easily for onsight or redpoint attempts. Release the spring by pushing the button on the bottom side of the handle and the central bolt moves easily and the Zap-O-Mat blocks quickly. Ideal for working routes or when the climber is heavier than the belayer. When lowering, the integrated karabiner arrester prevents the unit from being pushed too far thus increasing the braking power. The perfect device for the safety and comfort conscious rock climber. Useable with single ropes between 8.9 mm and 10.5 mm. One more detail.. The Zap-O-Mat is apparently a "hybrid tuber unit" (WHAT!) and not a semi-automatic belay device. Just in case you were wondering
Weight 158g

You can get an idea of how it is used by going to the following link:
Edelrid Zap-O-Mat