For real semi-auto pistol, ejection of spent case is powered by backward force generated while the bullet is fired.  In model gun, as no bullet is fired, the blowback force comes from a different way.

Basically, 1 gram of fire powder (known as Cap) is placed inside a model gun cartridge.  The barrel of model gun has a metal insert (known as Detonator) attached.  So when the cartridge is chambered, the front and end of cartridge will be support by the detonator and slide.  Explosion of Cap will push both front and end away in opposite direction.  As barrel is fixed with frame, moving parts (slide) will be pushed backward, simulating the blowback action of real semi-auto.  Same mechanism applies to full-auto model gun.
 

Cut-away view of Marushin Plugged Fire Catridge (PFC)

This is a cut-away side view of a model gun cartridge in chamber.  Cap is placed in the middle of cartrige which is sufficient to simulate blowback action of real gun.  The cycle are as follows:
 

Stage 1

The return of the opened slide sends one cartridge into the chamber.  Noted that the detonator inside the barrel is pushing the valve (inside cartridge) into ready position.
 

Stage 2

The trigger is released and the hammer is hitting on a firing plate mounted in the slide.

 
Stage 3

 Since the firing plate is pushing the cartridge further forward, the Cap is ignited by a pin inside the cartridge.
 

Stage 4

Ignited Cap produces massive pressure to the valve, forcing slide to move backwards.

 
Stage 5

Fire and smoke get through the valve and expelled through barrel.
 

Stage 6

The extractor in the slide pull the cartridge backwards to a point where the cartridge hit the ejector, forcing the cartridge to be ejected from the gun.  The whole cycle repeats.
 
 
Full-automatics

Full autos can be differentiated to Close-bolt and and open-bolt.  For Close-bolt autos, it works similar to Semi-automatics.  In full auto firing, the hammer will not be locked after a cycle when trigger is depressed.  Open-bolt is even more simpler, the detonator is set longer then normal semi-autos, so as soon as the round is in chamber, it will be ignited straight away.
 

Revolvers

You can say model revolver is least interesting.  In fact it is as the turning of cylinder and hammer (in double action revolver) is powered by  trigger only.  So the duty of Cap is just to provide sound and spark.  Model gun collectors usually love revolvers.  Reason? Just like when you have enough beer, you will turn to wine.
 

As you can see, the blowback action of model gun simulate closely to real gun's action.