A pictorial comparison of the POF-USA and FN FAL gas systems. Same deal with the last picture post: Go here
and click on the images to get the ginormous originals.
The POF-USA gas system is below the FN FAL gas system.
Here you can see the fitting that allows access to the gas system for cleaning on the left, and the piston to the right of that. On the FN FAL the piston and actuating rod are one piece and the system is tuned with an exterior dial (that, like the amp in "This is Spinal Tap", goes to 11) behind the front sight post. The POF-USA system uses a seperate piston and actuating rod allowing the piston to be turned around with either the 2 ring or 3 ring side toward the gas port, thereby tuning the system. I haven't played with it to see what difference having it one way or the other makes. Someone with POF-USA could probably give a better answer to that in any case.
This is the bolt carrier end of things. The dark rod off to the right of the FN FAL carrier pushes against the buffer spring in the stock and acts as a handle to pull the bolt carrier from the receiver. The FN FAL uses a spring around the actuating rod to help return the piston to it's initial position. Notice also how the FN FAL carrier has an integral carrier key, the part that the actuating rod contacts to move the carrier back, unlocking the bolt. This particular POF-USA carrier key is held on with two socket head screws, but some folks who were at the Shot Show mentioned seeing POF-USA systems utilizing integral carrier keys as well. The POF-USA system doesn't have a spring on the actuating rod, but rather relies on the buffer spring pushing the bolt carrier forward to return the actuating rod and piston to their initial positions.