If you pull 3 mobs then continue to run to get other 3 mobs I will put a regen after the first pack. If I don't your health will be at mid when you get to the second pack. As a WHM then I will 3x holy which will stun them. Plenty of time for you to get the agro back.
I do agree doing regen before pre pull of first pack is silly. Wait until first "flash" and then do it.

(WHM main)