Hi, I'm not a native english speaker, so I might get wrong understanding about this hunter ability ... My question is, why there is a 30% damage reduction added to Deterrance, if deterrence already provide you with immunity to all damage? Like if Mages Ice Block would have addittional 30% dmg reduction?? There is no sense at all. That's how I understand the word "deflect" all melle, ranged and spells attacks. Doesn't that mean something like avoid or parry or dodge all attacks ? If not, help me understand (translate) this word... ty.
It means that any damage that does go through Deterrence (such as DoTs that were on you before you used Deterrence or AoE magic damage that is not deflected) is reduced by 30%.
Deterrence doesn't provide immunity like pally bubble/shield or Ice Block, it just increases dodge and parry chance by 100% and gives 100% deflection about single target magic damage strikes, as far as I understand it.
The answers so far are correct, to be a little more specific though, there are some raid boss abilities (or DoTs as mentioned already) that deterrence won't make you immune to. In these cases it will still reduce damage taken by 30%. As far as what abilities it reflects completely, there's often no way to know besides asking someone who's tried it or trying it yourself and finding out. As a general rule though, really hard hitting boss abilities that are intended to pose a healing challenge won't be able to be reflected (still reduced 30% though) but mechanics like Ionization on H Jihn'rok that are meant to be blocked will work, this isn't always the case though so you usually just have to try it out.
The rule for what is "immune"'d and what is not is so damn random and is mostly just balanced by Blizzard so Hunters aren't too "good" on some fights. Arguing that Mages have Ice Block and Paladins have Bubble is not the same, they are both on a 5 min CD where Deterrance can be 1 minute CD.
There are some heavy boss abilities that you can completely immune yourself to with Deterrance:
-Tortos Breath(can save repair bills with Feign Death ;D)
-Dark Animus Full Power(Heroic prog. clutch kills)
-Iron Qon ice bombs, you can make the entire raid not take the AoE blast if you have Deterrance up before the debuff runs out.
-Twins(HC) Inferno Storm
-Lei Shen Static Shock solo soak
It is just very random on what it immunes you to and what it doesn't. I didn't expect Magaera Rampage to be deflected completely, helped a lot on our first kill to negate some of that massive damage
Also if you're curious to why they added the damage reduction, it was because during cataclysm affliction warlocks and spriests were killing hunters through deterrence. God that was really an awful time xD