Here's how to perform the ultimate target switch and a true global.  The reason these steps are important is because healers can usually heal through your damage, but they can't heal through a global:

1.  Target caller calls out a countdown on voice chat starting with 3.  "Switching to RandomDouche in 3...2...1...Attack", where 'Attack' is '0 time.'  Or use any word instead of 'Attack' to symbolize attack the target.  But, that doesn't mean you start switching to the target on the word 'Attack'

2.  DPS activiate damage increasing abilites and trinkets on the 3rd or 2nd second on the countodwn (unless they are tied to a specific damaging ability that you want to use on the target when you hit it).  

3.  DPS that use ranged cast times begin casting before time 0 on the countodwn timed to land at 0 time.  For example, if a DPS ability has a 2 second cast time, start casting 2 seconds before 0 time.

4.  Melee that need to run to the target begin running at the 3rd or 2nd second, unless they are already right next to the target.  Sprint if available should be used if necessary.  Time it so you are at the target at 0 time.

5.  Use ranged CC's on enemy healers up to 1.5 seconds before 0 time; just make sure you're not on cooldown at 0 time.

6.  Melee that can ability move to the target (like warrior intercept) activate the move at roughly 1.3 (global cooldown) seconds before 0 time, or whatever their individual global cooldown time is.

7.  All DPS instacasts on 0 time.  Casters follow up casted spell with instacast at 0 time.  Melee uses hardest hitting ability that doesn't require combo points.  It dies.

This creates a cascading effect where different abilities are begun and activiated at different times but all damage hits the target at 0 time.  The only exception to this is warriors for example, moving to the

Notice this is not waiting 3, 2, 1, and then start casting on the target.  You are already casting around 2.  This doesn't mean start finding the target at 0, or moving towards the target at 0.  At 0, everyone is instacasting, and there should be some spells and abilities already en route to the target.

This achieves a true global.  

Why do all of this?  Healers are quick but can only be so quick, and usually look at damage as an indicator of who needs to be attacked.  If the person is killed during a global, nothing can be done.  Now, healers that see everyone switching will probably put a damage reduction effect that can really hamper DPS and save the life of the target.  These are usually instant casts and that's why it's so important to kill the target before the healers can respond and start building shields and HOTs.  

We will practice this.