Last Collide

Diablo II Notation Basic Class Skill Item Ally Quest Area Monster Tale Attribute Unit Animation Attack Damage Death Size Radius Light Radius Melee Range adder Missile Chill Drain Damage regen Prevent Heal Altering AI Blinds Target Monster Flee Shrine Curse Aura

Willowisp lightning bolts are size 2 and have Last Collide enabled, so although they aren't destroyed on collision they normally only hit the Amazon once (as indicated by equipped items applying 100% Chance to Cast Nova on Striking). However, when a Decoy is summoned beside her, it then becomes the last target and the lightning bolt can collide with the Amazon a second time.