I switched to Norwegian style a few years ago and this is a question I ask myself anew every time I need to pfb.
There is an NP group on Ravelry and someone recently asked this very question. There was no great answer, just a link to a regular purling video. (:shrug:)
Personally, this is my least favorite increase whether I'm knitting NP, continental or English, but here's how I do mine:
-Purl the front leg as usual.
-but Don't pull the stitch off the needle!
-Position the right needle behind the back leg.
-Purl as usual including pulling the finished stitch off the needle.
Basically, it's the same directions for a regular pfb except the yarn is coming from the NP direction.
I think this increase results in an little odd little stitch that's just sorta hanging on. So I could be doing it incorrectly. I would love to know if I am doing it correctly, so I hope someone else with more experience will comment here. :waving:
Let me know if you don't understand my instructions and I'll try to post a video of this. Theoretically, my camera should be able upload to the interwebs.