The Schnellfeuer, or Model 712, was Mauser's answer to the Spanish production of selective fire C96 lookalikes. Just over 100,000 of these pistols were made by Mauser in the 1930s, mostly going to China (although some did see use in other countries, and also with the SS). They use 10- and 20-round detachable magazines, and are almost all chambered for the 7.63mm Mauser cartridge. Rate of fire is about 900-1000 rounds per minute.
One of the urban legends that has grown up around these guns is that Chinese soldiers would hold them sideways, and use the recoil to fire in a horizontal arc. This does work, but is a pretty crude way to use the gun. Without the attached shoulder stock, it is much better left on semiauto. With the stock, it makes a surprisingly effective and controllable submachine gun.
Thanks to TFBTV for the opportunity to shoot and film this very cool gun!