This is related to locks AND healers (written by me, not copied and pasted)
When in instances, I get it quite a lot that I am healed by the healers after I life tap. I'm not compaining, bit I'd like to give the explanation:
Warlocks has a ability to drain life which transfers life from your target to yourself. With the right spec, you get a +% bonus for each affliction effect on the target. So, drain life is really a very usefull mid-range damage effect that can get quite strong in conjunction with affliction. So, what I normally do is put +-3/4 affliction effects on the target and then drain life to get a descent bonus. But why drain the life when you are at full health? Then its basically just another DPS mechanism. So, before I drain life, i usually life tap before i drain life. For those not in the know, life tap converts a certain amount of health into mana. So, it goes:
Put affliction effects on target
Life tap to get mana back used for the first step (but now i lost some life)
drain life to get life back
Using this formula you kill a mob and come out with almost full health and mana. IE, less downtime.
So, when in instances, I get healers healing me after I life tap. I'm not complaining, but when we get in a tight spot where the healers must conserve their mana, just bear in mind that a lock losing health is not necessarily being attacked. So healing him/her is not always necessary as the next step in the process is to regain that life that was just lost.
On the other hand, if the lock does have aggro, please don't think that drain life will be of any help. At all. The channeling can be interupted. So in that case, fire away with everything you have.