Gas Blowback (GBB)
Gas blowback (GBB) guns are more common than non-gas blowback (NBB), as such you will see pistols and rifles that act very realistically. The blowback feature, in gas powered guns, uses some of the green gas, CO2, or propane to cycle the slide of a pistol or bolt of a rifle in a realistic manner. Not only does this provide a decent amount of recoil, but it also loads the next BB into the hop up chamber.
Pneumatic and Mechanical blowback
With AEGs there are two common forms of blowback, the pneumatic blowback and the mechanical blowback. The pneumatic blowback functions in a similar way to the gas blowback, however, a little of the air that propels the BB forward is also used to move the fake bolt back. A spring then returns the fake bolt to its forward position. Mechanical blowback is generated by a mechanism on the top of the gearbox that moves back and forth with piston inside the gearbox. The force that is generated does create a slight recoil for added realism. Once the piston is propelled forward inside the gearbox, a spring attached to the fake bolt moves the bolt back into its original position.