The 1W LED bulb screws into the unit in place of the standard halogen bulb. It gives a much better light than the halogen bulb without draining the battery, having tested it in the field it is, like most spot beams, not the best light for general caving (the 14 LED unit is much better) but it is a vast improvement on the halogen bulb for looking up or down avens.
At a cost of £15.90 each this is a good addition to make to the Petzl Duo 15 LED.