I don't know if it increase leech, but it certainly increase the healing you get at either the start or end of each round. This isn't very useful NORMALLY, because except for earth, it rarely gets more than 5 rounds, and at that regen only works if your char is damaged, and usually a char isn't damaged or go from damaged to dead within 2 rounds, so that's how much hp you'd get, 2x of the value.
That said, this is very valuable for survival trial, especially if you are not using good healers. You can sometimes kill all enemy except those that can't really hurt you and then drag it out (works best if you can generate clone/shield to absorb new damage or if enemy doesn't attack at all), this way, your char will regen to be in a better state for the next fight.