The Eldar use haywire grenades for disabling enemy vehicles. A haywire grenade sends out a powerful, short-range electromagnetic pulse which shorts out electrical wiring and disrupts the energy systems of its target. Haywire grenades may only be used against vehicles. A model attacking with a haywire grenade may only make a single attack, regardless of their Attacks characteristic or whether they charged. If the attack hits, roll a dice to determine the effect of the haywire grenade: 1= no effect, 2-5= glancing hit, 6= penetrating hit. A haywire grenade may only be used against a Dreadnought if it has already been immobilised.
Haywire Grenades
Source: Codex: Harlequins, p. 11