Backstab bonus damage will be applied on a duel victory against a full health target, regardless if it is offensive or defensive.
Backstab still works even when the attacker stunned.
Although Ambush has a different name and picture, it is considered to be the same ability as its effect is similar. Thus, it cannot be used at the same time as Backstab.
The bonus only applies when the target is at full health so Rogues with the power must be the first to strike before anyone else. Having full health at mid-battle, especially in the Arena, is very rare and would mostly indicate a very tough opponent (if he/she is still undamaged). The power is best utilized at the start of the battle. Excellent and fast coordination is a must if the team wants to optimize on the small window of opportunity to really make this power useful.
Due to the limited opportunity for best using the ability, Backstab is a hardly useful ability. It is recommended to skip buying Ambush and not to try the Annihilator chest just to land the power.
If you do get it while rolling for other things (like trying to get Zurran or Elaida), don't go out of your way and try to maximize it's potential by coming out of the gate and unloading your initial tokens before anyone else. Sometimes, it's best to wait and see how the other team play and adjust from there. This is especially so in the Arena and Festival battles.
The picture in the ability is seems to be the weapon Drakken Blade.