This is the best entity a map maker could have been given and there very simple to use, our 5 minute guide will have you use to them in no time so let's go.
What does it do?
Well it stops the engine from looking through the doors into other rooms which will help keep your level's r_speeds down a minimum.
I had a room which was getting r_speeds of around 400 at one point and once I'd put areaportals on the doors the speeds never went about 150 :))

1.Create 2 rooms and cut a hole for the door way in between them (place your player start and a light or 2) your door in and assign it to a func_door and give it a target of fa1.

3.Now here's the tricky bit :) in the middle of your door place a brush which hangs over the edges slightly like the image shown here

4.make sure your brush doesn't stick out off the front where it can be seen, give the areaportal these bit's of info


5.There that's it, what will happen now is when the door is opened the areaportal will disappear until the door closes greatly reducing your r_speeds