This can be a bit confusing, so PLEASE, if there is anything about my description you do not understand don't hesitate to ask for clarification.
Basically, every time Val heals 100% worth of a Hunter's health without that hunter becoming incapped will count as one point toward this mastery. If the hunter is incapped or killed at any point before 100% worth of health points have been healed, the progress toward that one point for that hunter is reset.
So for an in-game example, lets just say the hunters have exactly 100 points of health. Markov takes 60 points of damage, so now he is at 40/100 health points. Val heals Markov those 60 health points so he is back at 100/100. This does not count as a point toward the mastery. Then Markov takes 50 points of damage and is now at 50/100 points of health. Val heals Markov for 40 points so Markov is now at 90/100 health. This counts as one point toward the mastery, as 100 points of this hunter's health have been healed without the hunter being killed.
This is tracked separately for each hunter in the game, so heals on one hunter do not count towards heals on another hunter. (Ex. Hunter A has 40/100 health and Hunter B has 60/100 health, healing both of them to 100/100 health will not give you a point toward the mastery, but as soon as you heal Hunter A another 40 points and Hunter B another 60 Points, you will have 2 points toward the mastery, one point per hunter)
If the hunter is incapped while you have for example 60/100 heals done for them, it resets to 0/100 heals done for them and you have to start over on progress toward that hunter.