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