The way the talent Flameglow works is it uses your spellpower as a flat damage reduction to any hits that you take, with a maximum of 30% reduction of the hit. The spell power flat reduction formula is your Spell Power divided by 5 (may change again as PTR goes)
So as an example,
let's say you have 10000 spellpower (for simplicity purposes). Then you will have 2000 damage reduction.
If an incoming hit damages you for 6666.666 damage, you will take 4666.666 (2000 damage reduction)
If an incoming hit damages you less than 6666.666 damage, you will take only 70% of the damage (due to cap 30% damage reduction, therefore you are not reducing hits by exact 2000, otherwise you will take 0 damage from hits less than 2000 damage)
If an incoming hit damages you for 10000 damage, you will take only 8000 damage (2000 reduction, 20% of the actual hit).
For that reason, in order to use your spell power full amount of damage reduction (maximum use of spell power as flat damage reduction), the damage you take will have to be higher or equal to 66.66% of your spell power. Any hits below that number, you will gain 30% damage reduction, but not using your flat damage reduction potential. Also as the damage gets higher, the damage reduction remains flat, and so the % reduction becomes smaller. It's not a flat 30% permanent damage reduction to all hits.
Since this damage reduction threshold can be increased by increasing your spellpower, picking this talent means you can use your offensive abilities/items that increases your spellpower (trinkets, etc) to gain better passive damage reduction for the duration.
It will be interesting to see how the reduction works against damages from dots though.
Correct me if my Math is wrong, i am not too good in it :-}
Edit: corrected information, they added coefficient of (SP/5) as the damage reduction value
Thanks Emeka (poster below), i have changed the whole post to reflect it
Edited by Milktea on 2/6/2013 5:40 PM PST