The volumetric spotlight effect visualizes the physical effect of light reflecting of dust particles, fog or damp in a spotlight. The illusion of a volume is created by filling the cone with a series of planes to make it "volumetrical".
The shape is controllable by setting parameters of its width and length and you can choose to have it square. The color can also be set.
Since the volume is rendered by additive-transparant planes perpendicular to the spotlight (set by a parameter (default 512)), the render cost is very high, because really the entire volume has to be rendered, while 3D-rendering usually just renders surfaces. To prevent the worst case conditions, the number of planes are dynamically reduced depending on the distance to the camera. All performance settings are configurable. Please note that this effect will only render at a decent framerate (30FPS+) on mid-end and high-end machines.
The cool thing about this light is that it also supports (dynamic) masks. These work by just inserting a 2D mask at the correct position and transformation in the material shader that is rendered over the plane. So only 2D masks perpendicular to the lightray are supported, but simple geometry (Example a sphere) renders completely correct in 3D. This can make real convincing effects such as light through windows or panels, which makes the volumetric effect really strong and much cooler than the simple facing-plane god-rays.