It is a rather simple mechanic:
Originally Posted by bling581
She bites the #1 on Total produces Threat which is not a tank, and not linked to the tank.
1. Healing produces x0,5 Threat compared to Damage, so if a Healer does 2x as much Healing as the top DPSer did Damage, the healer will get bitten.
2. No -threat effect seems to work. We tried Mirror Image, Feign Death, Feint, Fade... she ignores such effects, threat only counts up for her.
3. Passive threat-reductions don't seem to work either. Rogue's inherent -20%, Cat Druid's -20%, -30% from Ele Shammy talent... I don't think they work, due to comparisons who she bit between healers with no -threat talent and DPSers with -threat talent.
So in short:
Take all damage done, take all healing done and half that. Sort the bars, whoever is highest and not a tank gets the initial bite.
-- (The Death of Rats, Terry Pratchett, Soul Music)