This weapon was only available to Seasonal characters during Season Four, but once this Season ended, started to drop for normal characters as well.
The unique affix, as of 2.4.2, causes Spike Trap to damage any enemy its blast (and only the blast, not secondary arcs of Lightning Rod) has hit again after 1 second, for the same damage, without any other effects that can trigger on a blast. It does not cause the trap to go off twice; the damage is simply duplicated again on each affected enemy, and does not chain further unless a new trap is actually detonated.
Stats (Level 70)
The Demon's Demise
Legendary Hand Crossbow
“Whispers abound of Kovan's insatiable lust for slaughtering the denizens of the Burning Hells. May the gods save the world if the day comes when he no longer has demons to fight” — Tales of the Wasteland
Prior to 2.4.2, which changed the functionality of Spike Trap, the unique affix caused Spike Trap (with Sticky Trap rune only) to spread to enemies the explosion hits. The original target wouldn't gain a new application upon detonation. This effect might chain repeatedly, being especially lethal to large hordes of foes.