He quotes from WoWBlues:
Gaining Power (Mana / Energy / Rage) also causes threat in most cases, for example taking a healing potion, or gaining rage from Bloodrage, or Energy from Thistle Tea.
For normal abilities, each point of Mana is 0.5 threat, Rage is 5 threat, and Energy is unknown, probably 5.
These forms of buffs all have infinite range; they will cause threat to all mobs on whose threat list you are on. Furthermore, the threat caused is split equally among all the affected mobs.
Note that threat caused from Power Gain is not affected by threat modifiers. Gaining 1 point of Rage will give 5 threat whether you are in Battle Stance or Defensive Stance.
For me, this is particularly interesting as my typical pre-pull setup is to Berserker Stance -> Berserker Rage -> Defensive Stance -> Bloodrage -> Pull
What this means is that I could actually generate more rage by doing the Pull BEFORE I hit Bloodrage. Doing so would help generate a bit more threat on the initial pull