Before I bought my PS90 I assumed the disc-like structure at the rear end of the magazine somehow rotated to turn the rounds into the feed position. However, I found that it was just a simple, helical, curved ramp with no extra moving parts. There are two rollers, but they just simulate rounds and assist pushing the real rounds.
So it is not some delicate rotating mechanism, just a helical ramp that does not move or break. It is very reliable and quite ingenious. It allows a very high ammo capacity without adding to the profile of the gun.