Spinning Lights discussion (cgtalk)

Seems to be explained how to use it very well here. Well worth a look.

Links also to this one:
Spinning Light Rig v1.1 on Newtek forum:

"Well, skylight rigs are the "next best thing" to rendering with global illumination (A.K.A. Radiosity). It simulates the effect of light coming from the atmosphere. The sun will give you direct lighting, but the sun also illuminates all the particles in the atmosphere, and this light bounces back to us, giving us higly diffused (un-focused) shadows. So if you have a blue sky, set your sky lights to be the same colour then add a sun (key) light and you got a quickly-lit outdoor scene. Radiosity gives you the effect of bounced lights, so that will have a quality edge over this rig. But the render speed more than makes up for it. Radiosity is many times slower."