Ambush takes effect when attacking an enemy from behind, or against any enemy which is not targeting you, and applies to both gun and melee damage. This makes it effectively a less powerful but more flexible version of Backstab. However the bonuses do stack with Backstab, so placing points in both skills is viable for a Bloodshed build.
All attacks from Decepti0n will receive the Ambush bonus.
As it takes effect simply by having the enemies target someone else, when there are teammates present, Ambush can usually be taken advantage of continuously simply by standing further back than your allies, ensuring the enemies target your closer allies, preferably a tough tank like a Gunzerker or the mechanical summons of the Commando or Mechromancer.
This skill synergizes well with Salvador's skill, Come At Me Bro. This skill causes all enemies to have an increased chance to target Sal, allowing Zer0 to use Ambush.