Lightning pulses periodically between all wearers of this item, dealing 1500% weapon damage.
This means lightning will arc between wearers of the ring, i.e. if you and your follower were wearing it, or two (or more) heroes were wearing it, arcs of lightning would go between them, damaging mobs hit by the lightning.
If your follower is the only one that has it equipped, the reason you’re not seeing it proc is because there’s no-one else wearing one for it to arc to.