It would be nice if they also made one of these, but a "light" version without that huge tank at the top, and just leave the M16 mag feeding mechanism in there so you can use M16 mags with this... that tank at the top is ugly as sin.
That aside, the way it works is simple. Are you familiar with the inner workings of a hicap? Yes you need a hicap to power this; basically this big box at the top, all it does is act as a resevoir to allow you to hold an additional 1200 bb's.. SO tecnically you dont need to put ANY bb's into this resevoir at all. You can work this with just a plain M16 mag of any kind... even a locap if you wanted to.
The way it works is that there is simply a path the bb's go from the M16 mag directly into the firing chamber of the P90. Think of this as a conversion kit to use M16 mags with a P90... And if you use a hicap, leave the back port open when you load it into the box mag, that way you can load an additional 1200-some odd bb's in there to further extend your ammo capasity... The bb's in the resevoir will just drop into the M16 hicap to refil it.