A: Here is a very simple diagram that I drew (go Autocad 2002!!!) comparing the two so you can get the main idea. These are views of the powerfeed and standard feed bodies looking down the barrel into the breech. The white arrows represent the paintballs going into breech. The red arrows represent the blow back gas; big arrows represent the majority of the blow back gas as opposed to the small arrows.
You can see on the standard feed, the majority of the blow back gas is vented right up the feed tube instead of the small hole drilled into the top of the feed tube. This means that the balls are pushed out of the feed tube in the opposite direction you want them to go. This can result in slower feeding and chopping, not to mention that paint can be blown right up into your hopper( very messy and will ruin all of the paint in your hopper ).
On the powerfeed, you can see how most of the blow back air is vented out of the rectangular slot cut into the top of the feed tube. Very little gas actually vents up the feed tube into the hopper. The air venting out of the rectangular slot actually creates a vacuum and helps suck the next ball into the breech. This means faster feeding, and if you get a bad breech break, the paint will volcano out of the hole stead of going right up into the hopper .
***(If you have Attention Deficit Disorder and can't read all that, a PF uses the leftover gas of the 1st shot to bounce the next shot into the breech faster than gravity can feed. *Halliday*)***
For those of you who will do something stupid, here is my AIM name ahead of time: vaypourus