This is a very useful technique for figuring out where to highlight or to paint source lighting..
If you want to know where to highlight a mini, or need to paint the light of a blazing fire on some armour or such, This is how you do it.
First, Buy a sunlamp like this one.
Then, Shape it like so:
Place a white cap on it (GW primer works perfectly) and turn it on.
Place a mini on it and see where the light hits it. (works better without a base.)
You can also tape the cap to it and face it down at a model for different angles.
note:- In the interest of safety remove the cap as soon as you've taken a photo or worked out the light angles.