Direct impingement means that the gases generated from firing a bullet are redirected through a gas tube. The gas tube feeds back into the upper receiver at the top of the bolt carrier group. When the gas pushes against the bolt carrier group, it causes it to eject the spent casing in the chamber and then feed the next round from the magazine.  A long-stroke piston system is basically a normal BCG with a long steel rod attached where the gas key would typically be. The piston goes into the same area of the receiver as a standard gas tube, but it’s about twice as large. When you fire a round, the pressure of the gasses pushes against the piston and then vents out through ports in the gas tube. This allows the rifle to run extremely cleanly and efficiently rather than basically vomiting all over itself after every shot like a DI gun. Another term for this is “piston gun.”  If you genuinely didn’t know that, I would highly recommend checking out this “How an AR-15 Works” video animation. It’s AWESOME. Also, check out “Inside Primary Weapons Systems…” for PWS specific knowledge. Also, if you’re brand new to the AR-15 world, I recommend checking out Palmetto State Armory. You can get a complete rifle for less than the price of this upper receiver. That’s not to say this is a ripoff, it’s just more of a high-end upper, whereas PSA is more entry-level.