How does health link work, exactly?

The Health-Link description taken from here says:

This trait is for champion monsters only and it activates only if more than one champion is present. Health Link reduces the amount of damage taken by the champion with that trait by linking his HP to that of all other Health Link champions in the area.

This trait can occur only for monsters equal to or greater than level 51

In short, any damage one monster takes is divided between all monsters in the link, so providing X monsters with health link stay near each other, it takes X times as long to single-target dps a monster with Health Link than one without.

It should be noted that if monsters are far enough apart, their "link" isn't active, so you can dps them one at a time if you can separate the group

Health Link means that the elite enemies pass some damage off to each other, i.e. if you focus one elite, the damage will be reduced because it is shared with nearby pals.

If they have no nearby pals, the damage doesn't seem to be shared. If you separate them it is possible to nuke one, and in practice because they spend some time separated so they will not die at exactly the same time.

So yes, it is harder to kill each one individually. However, if you use area-of-effect abilities you won't notice the difference.

For example, think about an ability that hits all three elites, and say anyone hit takes 33% damage while 66% is passed off to the other two elites (combined). So elite A takes 33% damage while B and C take 33% each. However, if you also hit elite B the same applies - he takes 33% while A and C take 33% each. Same goes for elite C, so added together each elite takes 33% directly, plus 33% x 2 from the other two elites passing damage off. Overall you still dealt 33% + 33% x 2 = ~100% damage to your main target.

This doesn't mean that area-of-effect abilities are better at dealing with them - it only means that you won't notice the effects of Health Link.

It shares the health of the linked monsters, and any damage done to any of them is taken from the pool.

For example: If a monster normally has 100hp each, and three of them are linked, they all share a pool of 300 health. You would have to deal a total of 300 damage to any of them, and then they would all die at once.

So the disadvantage is you can't kill them one at a time. You have to fight all three until their pool is drained and they all die at the same time.

As per the player's guide: "Health link reduces the amount of damage taken by the champion with that trait by linking his HP to that of all other Health Link champions in that area. This makes it much harder to clear an area of Champion monsters. When a Health Link Champion takes damage, that damage is divided amongst all other Health Link Champions in the vicinity."

Monster level minimum: 51