As a follow to the article on direct impingement, this article discusses the use of a piston driven gas system. A piston system is used as a replacement for DI because gas pistons are not recycling "dirty" gas back into the weapon like the DI system. The gas that is recycled with a DI system contains carbon and small amounts of unburned gun powder. Over time, this will collect on all the moving parts inside the receiver and cause the weapon to malfunction.
The piston system will still use the gas from a fired cartridge. However, it uses that gas to operate a piston which then cycles the weapon without introducing dirty gas to the receiver parts. This allows the weapon to operate for longer between cleanings and prolongs the life of internal components. Some of the piston systems have different pressure settings for shooting with suppressors. Piston systems are quickly becoming the standard on all gas blow back systems. They are found on AR weapons from LWRC, Ruger, and the kick-ass ACR. The video below demonstrates the difference between the two gas systems. I think the video is somewhat biased on the piston system because it shows the DI system failing. Both systems are very reliable, and the DI system has been used by the US Military for decades. Do not judge the reliability of the DI system by this video alone.