The vehicle's open deck is covered by a wide-area shadow field, enveloping the vehicle in darkness and hiding its true location. This has the effect of adding 6" to the range from enemy units wishing to fire at a vehicle with a Night Shield. This may put the vehicle out of range, in which case the shooting automatically misses. The extra distance is also counted for working out if the vehicle is within Rapid Fire range. It has no effect on template, ordnance or barrage weapons. The Night Shield does affect whether the vehicle can be seen in a Night Fight.
Night Shield
Source: Codex: Dark Eldar (2002), p. 13