Roller-delayed blowback operation uses rollers on the sides of the bolt head: as the bolt head is driven back these rollers swing out into slots in the breech and temporarily prevent rearward motion of the bolt head, while the bolt carrier is allowed to proceed rearwards.
The earliest roller-delayed blowback system was developed in the early 1940s by Michele and Bartolomeo Ortolani, who used it for a prototype submachine gun of their design. This gun was not successful but may have influenced later German developments. Towards the end of World War II, two Mauser engineers, Ludwig Vorgrimmler and Theodor Löffler, designed an early roller-delay system for the experimental MG 45 machine gun, and then implemented this system in the StG 45(M) prototypes. Vorgrimmler then developed this system further while working in France after the war, and later in Spain, where he used it in the CETME rifle.
Heckler & Koch of West Germany refined the system further in the late 1950s, when it produced the G3 rifle, directly developed from the Spanish CETME gun. H&K implemented this system in the many derivatives of the G3, including the MP5 submachine gun.