delayed blowback

More properly termed "delayed inertia," delayed blowback is the principle that describes the inertial operation of some firearms. Like most modern firearms, those with delayed blowback use kinetic energy from an expended round to force the bolt back. Through various means of mechanical disadvantage, these weapons delay the initial opening of the breech; thus, the delayed blowback mechanism(s) absorb a portion of the weapon's recoil, making them easier and more comfortable to use.

For example, the Heckler and Koch MP5 submachine gun uses what they call a "roller-locked delayed-blowback." When the bolt is closed, rollers carried in the bolt are wedged into receiver recesses. On firing, the rollers must be forced out of the recesses at great mechanical disadvantage, delaying opening of the bolt. Because much of the kinetic energy expended by a 9mm Parabellum round is absorbed by the motion of these rollers, one can fire many sequential rounds from this particular firearm without discomfort or any unfortunate accuracy problems caused by excessive recoil.

http://www.rkba.org/guns/principles/operating-systems/delayed-inertia.html

The primary reason to delay the blowback recoil operation of a firearm isn't to absorb recoil energy for the comfort of the user. At the moment of firing, the pressures inside the cartridge - and thus inside the chamber - are extreme, more than enough to split the cartridge wall and throw metal in whatever direction. The pressure rapidly drops as the bullet accelerates down the barrel (increasing the volume in which the majority of the gasses are trapped) and eventually the bullet leaves the muzzle, at which point the pressure drops dramatically.

The delayed blowback action is used when a firearm has a powerful enough cartridge that two things are true: one, the chamber must be kept mostly closed until the pressure has dropped far enough to permit the safe extraction of the empty cartridge and two, the recoil impulse is powerful enough that simple mass and spring tension isn't enough to perform this delay; some form of mechanical disadvantage must be used to further 'block' the chamber closed for long enough.