Ive not got the power and am going by info in Hero Builder which lists it as a PBAOE (play based area of effect), which means you must be in melee range of foes in order to drain their health and stamina and add it to yours.
As to how to slot or use it in conjunction with your primary, I can't really advise other than to look through the USA forums, someone there is bound to have posted help.
Try it and find out then you will be the expert and you can explain it to everyone here.
At a guess, if it takes accracies slot it with 1 accuracy first, but I think it may be an autohit toggle from the description, in which case probably best to slot it with EndMods and heals.
Most of these toggles actualy pulse, with the duration of the efect equal to the time between pulses. 1 second is a typical value. If you have any mobs within 5 feet of you at the time of the pulse your healing and recovery will be boosted until the next pulse. The amount very probably depends on the number of mobs in range.
This power probably also interupts Assassin Strike in PvP.
I really should do something about this signature.
This power requires you to be relatively in melee range, about 10ft from the mob.
It is not a toggle.
You cannot slot it with Accuracy enhancements (sux)
Has a slight debuff of health and end regen.
It is, as you've probably guessed, best used on groups of mobs (works well when teaming in big groups)
Has a base recharge of 120 seconds - 3 slotted for recharges gives just under 62 seconds, and with 3 slotted hasten 45.29 seconds (assuming they're all SOs!)
It is a regen, and therefore will not fill your bar straight away. The more mobs you hit, the faster + more endurance you will regenerate.
Personally, I have slotted it 3 recharge and 3 endurance recovery. I have never taken stamina, and never will on this character. (this power is sufficient for me). However this power get's mixed reviews anyway.
Best tactics, mass hold, drain psyche, and spam psychic shockwave when you get it.
@Under Control
You'll either love it or hate it.
I'm currently running a level 34 dominator (grav/psi). I don't have any recharged in it iirc, but I do have hasten. I tend to use it if my endurance bar is below about 25%. It's a click to use (not toggle), and it doesn't auto hit. With tactics, or a yellow inspiration, it will hit most enemies. The trick is to immob/hold a group of enemies, leap in and activate the power. If it hits 1 enemy, you won't notice much difference to either bar. If it hits 2, your endurance bar will start to increase regardless of the amount of end you're using (there may be exceptions to this). If it hits 3, you're all set, and your endurance bar will fill pretty fast. Also, your hp recovery will start to be noticable. More than 3, and you're rocking!
I skipped stamina, and i'm perfectly happy. There are also rumors that it will be modified to take acc enhancements.
Recharge and endmod SOs are good for this power.
Yup, I do wish they'd let us slot it for Accuracy though.
I haven't bothered adding onto mine as yet, so I'm just on a single recharge SO.
Gr8 power still in anyway though.
As suggested, Mass Hold/Sleep, pop in the middle of them and let fly with Drain Psyche, pop out and voila not much of a endurance or health problem anymore.
And I've noticed that with the -recharge on the enemies, they go down so much faster. 
Considering that I have Stamina and have it 4 slotted.
I still use Drain Psyche 'cause I spam holds like you wouldn't believe and my end drain although not too heavy does catch up to me when my teams got me in there dominating 3 groups (most likely running TK as well).
I'm a bit confused about how this power works. Because I don't know how this power works, I haven't got any idea how I should slot it. Please answer my questions below.
When I hit it, the power makes an acc check for all(?) enemies around me? My healing and end recovery is boosted for each(?) enemy it hits? Must i be in the near distance of the enemies whom I hit all the time to retain the recovery? Or should i run to a mob, hit it and run back?
Plz tell me everything I need to know about the power so I can slot it good.
Thx