This is an animation of the inner workings of the VM-68. Although the animation is of a VM, almost all open bolt blow back paintball guns work in a similar way. To get a better understanding of what is going on, I'll explain the operation:

1. When the trigger is pulled the secondary sear (which is connected to the trigger) pulls down the primary sear and releases the hammer.
2. The hammer is pushed forward by the main spring. While moving forward, the hammer pushes the secondary sear off of the primary sear. If you look at the primary sear on this diagram (#4), you'll see the secondary sear is connected to the primary sear by a little lip. This allows the primary sear to spring back up, ready to stop the hammer next time it passes over.
3. The hammer continues forward and strikes the valve; pushing the cup seal open. This allows air to flow through the valve and into the bolt (which in moving in sync with the hammer) and blows the paintball out of the 'gun.
4. After the hammer strikes the valve, some air also flows through the rear of the valve and blows the hammer back (hence the name 'blow back' paintball gun). The cup seal also closes immediately after this. Air pressure is the primary force closing the valve; the valve spring is mainly a guide. You can take the valve spring out of a VM and it will still work (usually).
5. When the hammer passes a certain point, the primary sear pops up into the groove in the hammer which prevents it from moving forward.
6. Releasing the trigger allows the secondary sear to move up and reset itself into the lip on the primary sear.
7. The cycle is now complete and the 'gun is ready to be fired again.

For those that are curious, I used Adobe Photoshop 5.5 and Adobe ImageReady 2.0 to make the animation. It took about 4 hours to create.