API blowback schematic (bottlenecked round)
Schematic of a rebated rim (lower) compared to rimless cartridge (upper). Note the rebated rim allows the bolt head to probe into the chamber with the round.
API blowback (APIB) is an open-bolt operating system where the round's primer is struck before it has been fully chambered, meaning that some of the recoil force is offset by the continued forward motion of the bolt group. Many open-bolt blowback submachine guns technically use this method, but since their primer ignition occurs very close to the action entering battery, the effect on their operation is minimal.
"True" API blowback guns tend to have an extended chamber to properly contain the forces involved: such a system allows for either a reduction in the weight of the weapon's bolt, or for a reduction in felt recoil if the weight of the bolt remains the same. This system was initially developed for the Becker Type M2, since simple blowback is unsuited to large weapons: the weight of the bolt required to keep it closed in the initial milliseconds of firing renders such a weapon impractical. The Becker, for example, would have required a bolt weighing a quarter of a ton to function as a simple blowback weapon, so heavy that the return spring would either have to be so powerful the gun could not cycle, or too light for the weapon to close its own bolt with the barrel elevated.