From BEAM Robotics Wiki
A pummer is a solar powered self activating LED flasher. Pummers are nocturnal critters that soak up sunlight during the daytime, then get active at night. Their nocturnal activity can either consist of a light show, or some pattern of sounds. The difference between a pummer and a BLFNAR (blinky light for no apparent reason) is that a pummer has a soft turn off, which means that it turns on fast, but slowly fades away.
Electronically, Pummers essentially consist of just two parts -- a nocturnal solar engine, and a load circuit (which is only powered at night). Currently the most common used Pummer circuit is one of the BEP Pummer circuits.