The Prometheus EG Recoil Spring Guide is a high quality guide that has both a rotating shaft and thrust bearing, ensuring that the spring can twist unhindered as it is compressed. This reduces spring fatigue, giving a more consistent energy output with less degradation over time.
The Thrust bearing acts to slightly pre-compress the spring, which will raise the fps by up to 3%. 
The rotating shaft is made from POM, which serves to reduce the noise from spring vibration between shots.