How does "Health Up" work?

My guess is that it is depending on the%. So you have 40% HP , getting HP up. The number increases but it is still 40% what you got.

It increases your max HP by 15% . So if one character with 20k gets the buff, it will have 23k. If a pesky Wiggs duo hit that toon for 20k while the buff is up, it will have 3k left when the effect wears off (assuming it didn't have protection). It's like temporary protection. Or that's how I understand it at least.
Assuming the description is right and the 20k toon is at 40% (8k), it will still get 3k health, since is based on your maximum HP and not your current HP.

"BrianBuchanans;912296" wrote:
I assumed it was all percentages. Maybe RG has 20k health, uses health up, ends up with 23k health. Whatever the percentage of health he has when the buff wears off, that's what sticks around. Maybe he takes 3k damage; right before the buff wears off he now has 20k/23k health, or 86.95%. Now he has 86.95% of 20k.

I believe this is correct. Health up is most useful right before you take damage that will affect health. It is a detriment if you apply it right before a healer heals, however. (If the heal is based on the healers health level and not the recipients).

Thanks... it's still not clear to me (perhaps I should try in a GW battle some time) what happens if a toon has already lost some health at the time the effect triggers or wears off. For instance, is it possible to have the following scenario:

Phasma has Health Up (20k +15% = 23k), takes 12k damage, triggers RG's taunt as it's now under 50%
Phasma take a turn, effect wears down, has now 11k health out of 20k (no longer 23k), and is now again above 50% health, can trigger RG's taunt again on next damage.

"Tiggus;914076" wrote:
Thanks... it's still not clear to me (perhaps I should try in a GW battle some time) what happens if a toon has already lost some health at the time the effect triggers or wears off. For instance, is it possible to have the following scenario:

Phasma has Health Up (20k +15% = 23k), takes 12k damage, triggers RG's taunt as it's now under 50%
Phasma take a turn, effect wears down, has now 11k health out of 20k (no longer 23k), and is now again above 50% health, can trigger RG's taunt again on next damage.

Well. Could happen with lifesteal.