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Backtrack belongs to the group of damage negating effects. It is not evasion.
This means it only negates damage, it does not evade any side effects of spells or attacks, like disables.
When an attack has e.g. Diffusal Blade 1 icon.png Diffusal Blade and Backtrack triggers on the attack, the damage is avoided, but the mana burn still takes effect.
This also means that Monkey King Bar icon.png Monkey King Bar will not stop Backtrack from avoiding attack damage.
Evasion has priority over Backtrack. When evasion procs, Backtrack can't proc on that attack.
Backtrack has the 3rd highest priority in the group of damage negating effects.
This means when combined with e.g. Aphotic Shield icon.png Aphotic Shield​ or Living Armor icon.png Living Armor​, Bactrack negates damage while the shield or armor do not.
As with all damage manipulating effects and despite the ability description, Backtrack can negate all 3 damage types.
Increases Faceless Void's effective HP by an average of 11%/18%/25%/33% against those.
Backtrack fully ignores damage which is flagged as HP Removal.