Health Leech is a mechanic that allows you to recover a percentage of the damage you deal as health.
Each time you deal damage (whether from a hit or damage over time), any applicable leech stats are calculated, then the amount of damage is multiplied by this value to get the amount of health you will gain.
For example, if you deal 100 physical damage with a melee attack and you have "5% of melee damage leeched as health" and "3% of physical damage leeched as health", you will leech 100 x (0.05 + 0.03) = 8 health.
The damage value used to calculate the health returned is the final damage dealt to the target, meaning defensive mechanics like resistances and block will reduce it.
If a target takes more damage than it has health remaining, the excess will not be used to calculate leech. For example, if an enemy has 25 health and you deal 100 damage to it, only 25 damage will be used for calculating leech.
Health returned by health leech is granted evenly over 3 seconds. Health from each instance of leech is granted independently, meaning they expire at different times. There is no limit to how much you can leech at once.