I don't have all the details on this yet, but I spoke with some people who have had their hands on it. Basically, you put an M4 mag in the well (top-right). Then you pull back the 'loading chamber' and pour in a bottle of BBs (Elite Force recommended).
Then you basically 'pump' the front end down like a shotgun and it loads about 40 BBs into your mag. So do the math: if you have a 120 round mid-cap, three pumps and it's done.
I'll talk with Elite Force/Umarex soon for more info. Pretty sweet item though.