Mana Break allows the attacking unit to burn a portion of the attack target's mana, and deal additional physical damage based on how much mana was burned. The Mana Break ability can be gained via items, passive abilities or talents.
The mana loss is applied before the attack damage is applied, and the damage is directly added to the unit's attack damage, allowing the unit to lifesteal off of the extra damage dealt. However, the Mana Break damage is not accounted by crits or cleaves.
Besides Anti-Mage's Mana Break, which does not stack with that of Diffusal Blade, all sources of Mana Break stack additively with each other.
Sources of Mana Break