It would be too hard for me to explain in words so heres a picture:

::Keep in mind this is all cylindrical::
So the green is your barrel, the red i guess would be a casing or housing. The black is actualy what your plunger is with the spring behind it. So the plunger is cocked at the front and is released and is pushed twords the back by the spring it pushes the air forwarward around and into the dart which is that grey block with the blue tip. The air evenly hits the dart around in a circle
A problem i can see with this is that you need a large diffrence between the barrel OD and the housing ID to get a big enough tank of air so your gun might be a little thick but i guess its a better alternative to having that ugly plunger handle sticking out the back.
So there ya go