Sparks

It's kinda like chain lightning, your first attack will have lightning extending from it to the next monster, which will then have lightning extending to the next, etc, etc, for a max of 3 extra hits, 1 per monster, at max. From my experience, extra damage from sparks always hits before the rest of the skill, like with Sharkwave, your first monster will get hit and spark the next few monsters, and then the rest of sharkwave will hit them.

 A couple pictures of sparks