It is the spped of bb, while its leaving the innerbarrel of replica. This speed is measured in FPS (Feet Per Second).

1 FPS = 0,3 ^m/_s

1 J = 99 ^m/_s

1 J = 326,7 FPS

Hop-Up system is used for overclocking bb during the shot (putting it in rotation), through which, it extends its flight path. Acting laws of physics make that bb lift up, while the gravitational force take it down. Appropriate adjustment of Hop-Up, will compensate for these two factors, which result in long and straight flight for longer distance and be greater focus shot (to improve accuracy).