This page attempts to explain how airsoft pistols work.

With the help of photographs and schematics, I'll attempt to describe the way in which air (or gas) is compressed to produce the power to project the BB.

Springers

Non Blow Back Gas pistols

Gas Blow Back pistols

Gas revolvers

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Springers

Springers are primed by pulling back the slide.

1. When the slide is pulled back the power spring is compressed.
2. As the slide is released into its normal position (to which it is returned by the recoil spring), a BB is collected from the top of the magazine and positioned in the receiver at the rear of the barrel.
3. At this point the gun is ready to fire.
4. When the trigger is pulled, the power spring is released, which pushes the piston through the nozzle.
5. Air is driven from the nozzle, behind the BB and down the barrel.

NBBs

Non-BlowBack gas pistols are all double action. They are primed and fired by pulling the trigger.

1. Gas is fed into a reservoir in the magazine or, on older/cheaper designs (eg Tokyo Marui's NBB range, excluding the MK23 and new 1911), in the handgrip.
2. When the trigger is pulled the barrel assembly is pulled back and a BB collected from the top of the magazine.
3. As the trigger is pulled further the hammer hits the gas release on the magazine (or in-built gas reservoir).
4. The gas is channeled from the magazine/reservoir, into the back of the barrel and then down to push the BB out of the barrel.

GBBs

Gas Blow Back pistols are the most complex, but most realistic, type of airsoft gun.

1. Some are Double Action, some are single action, depending upon the nature of the real-steel gun they are replicating. In the case of single action guns, they need to be cocked first, usually by racking the slide or a cocking handle on something like a Micro UZI or Mac 11.
2. This collects the first BB for firing and readies the mechanism for firing. On double action guns, this is achieved by simply pulling the trigger.
3. Once the gun is cocked, pulling the trigger causes the gas release (connected to the hammer in the case of the Beretta M9 shown above) to hit the gas release valve on the magazine.
4. The gas is channeled up to the power valve, which is pushed through the nozzle, forcing the BB down the barrel. (Past the Hop-Up in the M9's case, which puts back spin on the BB for extra range).
5. Once the BB is released, the valve blocks the barrel and remaining gas is used to drive the slide backwards, where it resets the mechanism ready to fire again.
6. The slide reaches the point where the recoil spring is fully compressed and then is forced to return to its normal position, as the recoil spring releases. As it does, it collects the next BB from the magazine and is ready to fire again.

Gas Revolvers

Most revolvers (spring and gas) work in a similar fashion. A notable exception to this is the PEGASUS system employed by Tanaka, where the gas and BBs are held in a cylinder which works like a magazine, rather than using shells - This webpage explains the PEGASUS system well.

Anyway, the majority of gas revolvers (which use removable shells) work thus :

1. Gas is fed into a reservoir, in the handle of the gun (note that this usually precludes use of real steel grips on airsoft revolvers).
2. BBs are placed in removable shells, which are placed, like the real thing, into the cylinder.
   Michael McCann wrote to tell me how the multiple BB per shell systems work :
   

   I noticed there is a question as to how revolvers can fire one bb from a shell with multiple BB's. 
   The answer is remarkably simple and surprising, and it's in their brochure! 
   The Shell itself has two skins, one which holds the bbs, and one which fits the gun. 
   The inner skin also has a spring which pushes the bb's forward. 
   Now here's the tricky part: The outer skin actually keeps the bb's from escaping by bending inward at the front. 
   Gas is released into the back of the shell, throught the hole. 
   Most of the gas is channeled between the outer and inner skin and acts on the bb in front, pushing it out and down 
   the barrel. 
   What little acts on the bb's inside the shell is only to push the first one past the outer skin holding it, and the 
   others into place behind it. 
   Pressure from the gas coming around the sides keeps the second bb in place as the first is fired off. 
   

3. Pulling the trigger rotates the cylinder one place, cocks the hammer and causes the gas release (connected to the hammer in the case of the KWC Revolver shown above) to hit the gas release valve on the reservoir - This is the case with double action revolvers (as most are).
4. It is also possible to rotate the cylinder and cock the hammer, simply by pulling back the hammer. Pulling the trigger will fire the weapon as before. This is a single-action mechanism, but is very rare in isolation, rather it's an option for better accuracy (and lighter trigger pull) on double action revolvers. - Spring revolvers are all single action and compress air in a reservoir, by cocking the hammer.
5. The gas is channeled up to the cylinder, forcing the BB from the cartridge and down the barrel. Sometimes this route can be circuitous and often has many places where gases can leak, making revolvers less powerful airsoft weapons, generally.

The following description is taken from www.hammermods.com - a great resource for Tanaka revolvers.

The Tanaka revolvers work on the PEGASUS system (Progress, Effective Gas Universal System)

1. Basically, this is just a catchy acronym to explain the unique mechanism Tanaka has created. Instead of storing gas inside the handle, gas is stored in the cylinder. (A)
2. The hammer hits a strike pin which will depress the valve (B) releasing gas to fire a bb. Because the bb sits right in front of the release, (and because it is a non-blowback) the Tanaka revolvers kick out plenty of power. As a matter of fact, Tanaka revolvers will put out more power than any GBB on the market with similar inner barrel length.