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Healing amplification is what the name says. It amplifies the amount of hit points you receive from healing spell effects.
For Monks and their ability to regenerate ki, healing amplification greatly stretches their effective use of ki in self-healing during an adventure. As you conclude your Adept training and spend more action points, all Monks (really, all classes) should work towards improving this ability.
Such amplifying effects include but aren't limited to:
Calculating your amplification isn't complicated with Update 24. You can now see your total healing amplification on your character sheet. Just hover your mouse over your HP to see it.
The principle of gaining healing amplification has not only been simplified with Update 24 but has greater effect.
If your character's a living creature such as Human, Elf, Dwarf, Drow, Half-Elf, Elemental or Animal, you have a Positive Energy Vulnerability (PEV) of 100. So, a spell or potion that can cure up to 30 points will cure you for 30 points.
If you are Warforged, your Positive Energy Vulnerability is 50. So a potion that cures 30 points will only cure you for 15 points. Bladeforged characters have a 40 PEV, so a 30 point potion works for only 12 points.
From here, healing amplification found on different items, enhancements or epic destinies can increase your total positive energy healing.
The Heal skill is normally a cross-class skill for Monks. Still, if you can find gears or care to use points there, you gain 1 to your Positive Healing Amplification for each point in Heal. This skill does not affect the results of the Wholeness of Body feat's healing ability.
Devotion effects adds to your Positive spell power, which has a stacking effect with healing amplification.
If you have a Devotion effect of 66, any positive healing you self-generate (from Healing Ki, healing spells and potion, but NOT vampiric effects such as Fists of Light) will gain an extra boost to your total healing. Devotion effects of different items do not stack. Devotion effects do not affect the results of the Wholeness of Body feat's healing ability.
Humans gain the racial Human Improved Recovery enhancement. This adds up to 30 to your healing amplification alone. So a Human with all of these enhancements will be healed 69 points with a 30 point healing potion.
Half-elves gain a lesser version of Human Improved Recovery but with 2 levels and not three, for a 20 bonus.
Update 19 omitted "Monk Improved Recovery" from the new enhancements tree, which gave healing amplification to any Monk.
As Monks go, a Human Shintao Monk gains the most enhancement-based amplification. Outside of the Half-Elf, no other Monk enhancements or racial enhancements provide innate healing amplification.
The Light Monk gets 10 to Positive Healing Amplification for each of the six core enhancements trained for a total of 60. So, a Human that has trained all three Human Improved Recovery enhancements and all Shintao core enhancements has a total amplification of 110. A 30 point healing potion will heal for 93 points.
The tier 4 enhancement Vigor of Life adds 20 to healing amplification. Training Harper Agent enhancements likely means you are building a very unusual Monk to spend 20 Action Points in this tree to qualify for this enhancement.
The Bath House amenity on modern guild airships give a stacking Guild bonus of +20 to healing amp for any character.
Two destinies add 20 to healing amplification: Exalted Angel's Purity of Essence (tier 3 ability) and Unyielding Sentinel's Vigor of Battle (an innate ability which requires you to be using the destiny; it cannot be used through Twists of Fate).
For all Monks, especially those with races or enhancements without healing amplification bonuses, there are quite a lot of items that heal you vampirically, stealing HP from whatever you hit. Vampiric effects aren't affected by healing amplification but can supplement your total healing.
Again, Devotion and positive energy spell power helps only with healing effects you create (that is, from Healing Ki). Devotion and positive energy spell power can't improve vampiric effects such as Lesser Vampirism or from Fists of Light.
During their life, Monks can seek out these items that will improve overall healing amplification.
Remember that Shintao core healing effects and racial Improved Recovery stack with effects from all of these items. All of these items give Competence bonuses to healing amplification unless otherwise specified.