As others have mentioned, it's an IE only trick for page transitions and is implemented using a meta tag in your headinclude template.
Try this page for a list of some of the different effects you can play with...
http://www.jansfreeware.com/articles...ansitions.html
I'm not a fan of it since it affects page load time (I'd rather the page come up ASAP instead of waiting for a transition) so while it is a neat trick I really couldn't recommend using it.