The PP-90 represents the abortive attempt to copy the concept (and probably some design ideas) from unsuccessful American Ares FMG (Folding Machine Gun) weapon, produced by Ares Defense company in mid-1980s. PP-90 had been developed in early 1990s by KBP design bureau in Tula, otherwise known for highly efficient and successful weapons, from submachine guns and up to tank and naval guns. Both Ares FMG and its PP-90 clone were intended for concealed carry, and folded down to relatively compact, plain looking metal box. When needed, these guns were to be unfolded into ready to fire position in 3 - 4 seconds. I'm not sure about Ares FMG, but the PP-90 became a failure - resulting design was somewhat unreliable and totally uncomfortable. Initially issued to some police and security forces in Russia, this gun now destined to collect dust in farthest corners of the armory rooms, due its terrible ergonomics and poor handling characteristics.
The PP-90 is entirely made from stamped steel, and folds around the point just behind the barrel breech face. PP-90 is a blowback operated, automatic only weapon (selective fired in PP-90M1 version).