What it does

"Fracture a random modifier on a rare item with at least 4 modifiers, locking it in place." You don't choose which modifier is fractured, and the orb can't be used on an item that is already fractured.

How players use it

A fractured modifier can no longer be modified in any way. That makes it the anchor for everything after it: Chaos Orbs, Orbs of Annulment and Exalted Orbs can then work on the rest of the item without risking the locked affix.

Getting it

Since patch 0.5.0, Fracturing Orbs are among the rewards that need specialisation through Atlas passive skills, so not every player is farming them.