The "tagging" rule is, if the mob in "green" meaning it can give exp to the higher level, then full exp is given to the lowbie who tagged the mob.

A level 63 mob is green to a level 70. This level 63 mob is orange to a level 60, so it will takes some decent +hit gear to consistently hit them to get "tagged" But at level 61 or 62, you can mostly tag the mob at first hit/cast.

A good spot for this is NorthWest in Zangarmarsh. the Greate Spore (flying thingy) is non-agro and plenty around. I used to have my 61 priest dot those, and have my ungrouped 70 "ret" pally finish em. The result is the priest gets full exp as if he "solo" the 63 mob. So it's a good grind. However, in my experience, questing still yields better exp/time. The time it takes to kill 30 of those mobs for a "bubble" of exp, I could have gotten as much doing a quest in less time. The only reason I grinded was that I needed to be 62 to obtain more quests in Zangarmash at the time.

But yeah, questing such as kill quest after level 63 was fast because I was tagging mobs with my lowbie while a high dps 70 was ungrouped and on follow--and sometimes it was faster to just group and AoE the area of interest.