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What IS power leveling? Power leveling in World of Warcraft is the method in which people literally bypass the normal questing route of going from zone to zone completing appropriate level quests and instances to rushing through levels by killing efficiently and doing quests in a certain way.
Some may go as far as having a high level run them through instances to kill all the elite mobs off having the character gain experience without fighting anything. There are many ways to go about it, but here are the most popular:
Guilds are the new power level machines.
With the inclusion of guild 'leveling' in Cataclysm, many have used the experience increases to outright speed a character's experience gain. Add that onto gaining access to heirloom gear from guild specific vendors, guilds themselves are a way to power level.
Instance with a high level.
Asking that high level guildie of yours to run an at level (or above level) for you is also a form of power leveling. Unlike in twinking where a under leveled alt sits towards the entrance to avoid gaining experience, a high level will normally run in and gather many mobs while the lower level hangs back not attacking until the high level has all threat.
The low level would be say a room away while the mass farm happens so that they can then leech the experience and have rights to roll on the item drops. This tactic can also be used in killing mobs outside of instances.
Kill mobs in groups efficiently with friends in high places.
Normally in farming mobs along, one goes for green level mobs because of the easier time in killing them. But if you have a high leveled friend, one can by pass this by grouping with and having the high level area of effect higher leveled red mobs, netting a low level lots of experience in a shorter amount of time. Doing this while questing will net the high level things to sell, and you faster quest completion alongside the experience gain.