It means that BM hunters can get 10% more extra Cobra Shot's dmg.
I already cleared this quest, so I bringed picture from one of Korean WoW forum.
Red Square is Cobra Shot and Green Square is Auto Attack.
The writer hit Cobra Shot 15 times.
Upper recount is non activate Tonga's eye. Upper one dealt 1.6M dmg
Lower recount is activate Tonga's eye. Lower one dealt 2.02M dmg
There are a bunch of toys and quest items that, for one reason or another, act as pets and therefore increase Cobra Shot damage... though, I don't suppose MMOC is the place to discuss this type of thing