Yes you are wrong saintseiya, this piece enhances your absorb spells (Absorb-STR VIT DEX etc) and makes them stick for an extra 10 seconds, it does not enhance your drain, drain II or aspir.
The reason why you got the HP boost effect is because that is what Drain II does, if you drain enough HP when you are at full hp or almost full, Drain II will add on extra HP giving you an HP Boost effect, this is only exclusive to Drain II, and is used in either making sure you stay alive, or with Souleater so you have that extra HP to not only boost Souleater, but the extra HP is there for when you sacrifice the HP due to souleater and if you was to get the hate and take hits.
Drain II is an awesome move and i often use it with my DRK Afv2 gears to boost it as much as i can :) I have had over 1800 before because i Magic Burst the move once in dynamis :D My monk friend was shocked lol
Simply put, this piece does not make Drain II give you an HP boost or enhance it, because it already does that anyways :)
There is only one downfall to this piece like the rest of the Black set, thats the -% damage stats, be sure to macro this out soon as you have done a weapon skill or get hate, drks have bad evasion (only 225 capped at level 75) and we get hit loads as it is, adding that stat is a death wish when taking the hits.
Besides that its an awesome weapon skill piece, and should be there every time you Guillotine, Spinning slash etc.
I just bought this great piece on Hades server for 790k, i cant wait to try it out on my weapon skills ^^