The device is a virtual device in a sense that there is no hardward associated to it.
Its job is that of a regular eggtimer, ie. do something for a specific amount of time. One little addition is the ability to restart the timer using a trigger.
The main use case is a timed motion sensor based light.
Motion happens and executes the eggtimer device. The device turns on the light for a specific amount of time. If motion happens again, the timer will be extended.
At the moment, the eggtimer device binary is only available on request. A 1204 binary can be found on deb. Copy that into /usr/pluto/bin, create an eggtimer device (Device template #2336) under the core, quick reload the router, and the device should run.