So TLDR what it pretty much does is it turns the environment lighting in to an area light which is much much more efficiently sampled in Cycles. Cycles is a monte carlo forward pathtracer which is very stupid in the way it does things so you have to help guide it along.... A lot.
Here is a little tutorial on how to use it (For now it will change in the future)
First you need an interior scene with light coming from outside (World light not any light from outside) or else this feature is pretty much pointless.
Second you need to add an area lamp where the hole in the object is.
Third make sure portal is turned on. This will make the area light no longer function as an area light so all the settings you see there are useless except for the size.
In the end you should have something like this. That black box around the light is to prevent the world light from going in the cracks(Im guessing) which would cause a crazy amount of fireflies. Dont worry this is automatically done no magic is required :D
Here is an image with light portals. 33 second render time
Here is a link to the original Blenderartists thread