Ambient = Dark

Diffuses = Light

An Ambient Occlusion pass returns a gray scale image with intersecting geometry darker. This technique allows you to do some post production using layer manipulation. Revit 2011 has some built in Ambient Occlusion capabilities so be on a lookout in the next release.

The below image was created via a script and I use this technique on almost all of my work.  It gives your illustration a more realistic feel. This is also how I create the museum board stills and animation.