OK, when round 2 of strand of the ancients begins everybody gets a debuff which prevents them from moving for a period of time.
Mages and paladin can ice block/bubble this debuff. This can also be done before you are even teleported to the new starting area for maximum effect.
One use of this is to give you an advantage in rushing towards one of the rocket launcher thingies so you can get there first and do the achievements.
AFAIK this does not work with the pvp trinket in place of IB/bubble.