Lightning is a utility program which can be used to knock out a guard for 2 turns; it costs 8 PWR to use, and has no cooldown. Lightning can only target a guard who is TAGged, and using it removes the TAG. Lightning can be bought from some Server Terminals for 800 credits.
To use Lightning, select it in Incognita view and then click on the TAGged guard you wish to knock out. Lightning can target any human guard regardless of their armor, even if the guard is already knocked out; it cannot target drones.