Gas-delayed blowback
Gas-delayed blowback (also called gas-retarded blowback) is a firearm operating principle (not to be confused with gas-operated). The bolt is never really locked, and so is pushed rearward by the expanding propellant gases as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt.
Gas-delayed blowback is one of the rarer operating principles. It is used by the HK P7 and Steyr GB pistols.