Double damage afix on the backpack

[Stan]

Title is pretty self explanatory, how does the 100%/300% damage afixes work on the backpack, does the item do double damage ? Do you take double damage as well, since the icon doesn't appear above your head ?

[kermat_]

The double/quad damage dealt is still applied, but this damage is still modified by the % of the corresponding backpack mutation.
You don't take double/quad damage when the item is in the backpack.