Deal no Non-Fire damage?

Can anyone explain to me what exatly does "Deal no Non-Fire damage" from the Avatar of Fire keystone?

It means that if you would deal physical, lightning, cold or chaos damage to an enemy, that damage is reduced to 0.

so, if my use a cold skill which deal 100 cold damage.
with avatar of fire, result deal only 50 fire damage. instead of 50 cold and 50 fire?

Yes.

i never before have used this keystone, now i think i will never use this XD

If you can convert the other half to fire, then you'll lose nothing.

For example, Infernal blow has 50% physical converted to fire. Alongside AoF, all 100% of physical would be converted to fire and you wouldn't lose any damage.

Cold to Fire is a pretty strong support gem, especially since it'll help prevent the loss of damage with AoF.

but if i thought covert 100% of cold damage to fire (avatar and cold-to-fire gem), then convert 100% of fire to chaos (with those upcoming stuff), this will result deal zero damage :D

now, my last hope is pyre.