Effect
Elemental damage from your weapon contributes to Elemental Exposure.
Elemental Exposure
Elemental Exposure rewards wizards who juggle several damage types. Each element you hit an enemy with applies its own vulnerability stack, and all four stacks together noticeably increase the damage the target takes from your attacks. Importantly, the elemental damage on your weapon also contributes to building the stacks.
The passive reaches its full value in builds that already cover multiple elements, for example through runes that convert a spell's damage type. With a single-color damage build you usually only assemble a fraction of the possible stacks, so the slot rarely pays off. Once you combine fire, cold, lightning and arcane in your kit, however, it becomes a cheap, gear-free damage multiplier that ramps up automatically during the fight.
Building all four stacks
Each damage type counts exactly once: hit an enemy with Arcane, Cold, Fire and Lightning and it carries four stacks of 5% each. Every stack lasts 5 seconds. So owning four elements is not enough; you also need to land them on the same target in quick succession. Further hits with an element that has already applied its stack do not add another one.
Since the passive only unlocks at level 68, it hardly matters while leveling. Its value shows up in the endgame.
Which partners supply the elements
Several common companions cover elements almost for free. Storm Armor periodically shocks nearby enemies with Lightning, keeping the lightning stack running without an extra button press. Black Hole pulls enemies together and deals Arcane damage, so an entire group picks up the arcane stack at once. Your weapon's elemental damage is added with every attack, which is why a weapon whose element your spells do not already cover is the smarter pick here.
Alongside Audacity and Galvanizing Ward
The passive often shares the bar with Audacity and Galvanizing Ward. Audacity only boosts hits on nearby enemies, whereas Elemental Exposure works at any range, so when you are standing in the middle of a pack both apply at the same time. Galvanizing Ward adds the protective shield that two pure damage passives lack.