Max HP Boost means that the maximum health, so called Max Health, of the target hero is increased by a certain percentage. As a result, they can take significantly more hits. The effect is not a Buff, it has no icon, and can't be purged. It happens instantly.
Max HP Boost does not extend the hero's health bar because the bar always represents the 100% value and the current Health is a certain percentage of it.
If the hero does not have full health, his health visually drops because the current value has now become smaller in proportion to the increased maximum health. Don't worry, this is only obvious, in fact the hero still has more hit points as before.
If the hero has full health, the Max HP Boost effect does not change anything visually. But the character has more hit points than before.
Max Health Boost can be used multiple times, but the boost is limited to a total of +100%, or in other words: the character can have twice as many hit points. Healers feel the effects of the Max HP Boost in the fact that their healing arts seem to be waning. However, this is only because the ally now has more health and can therefore withstand much more damage which the healer needs to heal. See Heal for an in-depth insight into the healing arts. Healer Runes are a good choice for Heroes with a large health pool, as these Runes heal a percentage (up to 10%, and 2x of that in CRIT Heals), but not just a fixed amount of health.
Dwarven Vitality (Trait): Self and Dwarf allies' Max HP is boosted by % and Lifesteal a % of the damage dealt. Dwarven EMT (Special Attack): Heals or Repairs target ally, boosts the ally's Max HP and grants the ally Haste.
Vengeful Dwarf (Trait): Whenever an allied Dwarf is killed, this Hero will gain [15%/20%/25%] Attack, Defense, Max Health and will make a basic attack on the enemy who made the killing blow. The percentage value depends on the rank (I-III) the trait.
Fillet Zomminions (Special Attack): Snacrifices all allied Zomminions. Self and remaining allies gain +1 Energy and +Max HP, plus +1 Energy and +Max HP for each Zomminion Snacrificed. Taste for Brains (Special Attack): Ranged Dark attack that purges all Buffs, heals self and allied Undead and buffs their Max HP. When dealing a killing blow or targeting an enemy corpse, the corpse is removed.
Chroma Shift (Special Attack): Ranged Physical attack on target, with a chance to apply Venom. 2x damage if target already has Venom. This Hero then shifts to target’s Element, cleanses all debuffs from self, and gains 20% DEF and Max Health until the end of the next round. Cleanses self again when shift expires. Starts powered, but cannot be used while shifted. Cooldown: 6 Turns.