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

Indications v2, v3, v6 does not mean the level of advance, but only shape of gearbox skeleton which was used and selected by the manufacturer depending on the capabilities of the body of the replica. For example, a models of the M4/M16 family, MP5, have a v2 gearbox, while the AK family have gearbox v3.

Battery capacity determines the time, which it accumulated enough energy to work gearbox. The greater  capacity, then replica will shoot longer.

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).

Blow back is a simulation system of the cocking lever - it imitates the work of it as the equivalent sharp. This system raises the realism and adds a lot more experience during shooting.

NiCd rechargeable battery is characterized by high efficiency current-carrying capacity, but there are disruptive effect on memory. Should be unloaded and loaded in the full cycle, ie, it is recommended to fully discharge, and after unloading must charge the battery immediately without leaving it for a long time in a discharged state.
NiMH batteries have a higher self-discharge over time and shorten the life - on the other hand, they earn about 30% more capacity.