Most ar 10s are overgassed. This means the gas port in the barrel is larger then required to make the rifle function . When u start loading up to higher end of powder charge this bigger gas port lets more gas in and unlocks the bolt before pressure at the case subsides. In other words it drives the case backwards, since the bolt is just slightly starting to cam open it gives a nice ejector print. I saw this on brass in my cmmg and was way slower then what it should have been, like 200 fps slow. The jp gas block lets u dial down port size, thereby decreasingport pressure/volume. U set it so that it just locks back the bolt on a empty mag after firing ur chosen load. Makes bolt movement less violent and somewhat softens recoil. The recoil part is subjective and depends on how oversized the port was.