The specifics is that whenever a unit with the "Power From Pain" kills off an entire enemy non-vehicle unit, the unit will gain a pain token which will give them benefits depending on how many pain tokens you get;
1 Pain Token: Feel No Pain
2 Pain Token: Furious Charge
3+ Pain Token: Fearless
The fun part about this? It can stack. So if an unit kills enough, it will have FnP, Furious Charge, and Fearless all in one go. But if this is not enough, they decided that "Hey, let's make them evil version of Care Bears!" and alas, we ended up with a sub-rule called..."Sharing the Pain".
Now Sharing the Pain is a rather interesting rule as they essentially allow pain tokens to be transferred around like some sick version of passing the ball, where instead of the ball they will pass instead pain for their opponents.
As stated by the codex, here's the "Sharing The Pain" rule;
"If a Dark Eldar unit is joined by one or more characters with pain tokens, all of the pain tokens for the unit and the characters that have joined it are pooled....If an Independent Characxter leaves a unit with one or more pain tokens, you must divide them as equally as possible. The owning player decides who gets any remaining tokens - just be sure to tell your opponent."
This point really tells us one thing; we are allowed to 'ninja' away pain tokens and give it to others in order to pump it up to augment its ability. For example a Wych squad in its basic mode is a good unit, but it suffers from lack of durability. So if I want this unit to go about being tougher, I should kill more. But that often means that some of your wyches will always go down before your squad gains the token, which in the long run can be rather bad for you. Why not get these tokens from somewhere else and get the token to begin with, rather than to risk harm upon oneself? An IC can help give pain token by joining the wyches to boost the wyches with Feel No Pain, which makes them being able to survive longer to give more pain to our dear opponents.
Likewise, the token will be pulled away if it is not truly needed by a squad to another squad. Now you must be asking ; Why would I want to pull tokens away from squads? Aren't I suppose to gain more Tokens? My answer would be that sometimes an unit will have tokens that would not suit its role, or that the token is seriously needed by another squad.
Take for example a squad of Kabalites with two pain tokens and an archon in it. Nearby this unit, there is another squad of Wyches who go no pain token whatsoever, but I know that it will need to survive long enough for my own plans to come to fruitation. What will I do? Basically I will pull one of the pain tokens from the Kabalites and attach myself to the Wyches and now both Kabalite and also Wyches will get their Feel No Pain, and also freeing the Kabalites from a pain token it does not truly need for the moment; Furious Charge. While the IC is at it, he is hiding amongst the normal fodders.
In its core concept it is a nice idea, but somehow you expect it to work in a way that it happens only in the middle of the game. Fear not, as this can be done during the early parts of the game, in fact I can do this on the first turn. But I do warn you that this idea works with a balanced army in mind, or with a Haemonculi Coven. So for those lists heavy on Kabalites and Wyches, it is much harder if not impossible to pull this stunt on the first turn.
The way I go about this involves quite a number of units but the essentials would be something like this;
1 squad of Wracks
1 Archon (preferably with drugs)
1 Hekatrix bloodbrides (can be replaced with any choppy goodness unit you want, but I find units with drugs work best)
My ninja trick is that I can potentially pull and make a squad deadly within seconds, with the second squad being able to benefit from a minimum of one pain token, and at max three pain tokens.
How the setup would work is that the Archon, Haemonculi and Wracks would start together on the table with the bloodbrides nearby. The Haemonculi and the Wracks together would already have 3 pain tokens to begin with, as each Haemonculus and the Wracks start with one pain token. If the Archon would roll a 6 on his drug rolls, he would already give himself a pain token, which will make the squad having 4 pain tokens.
Now the first turn is quite important for this, as if I forget this, I will be putting my Archon with a unit that would slow him down. What I am going to do is that (assuming that I only got 3 pain tokens, and not rolling 6 on the drug roll) I will pull two pain tokens from this Wrack/Haemonculi squad using the Archon and will proceed to join the Bloodbrides with the two pain tokens.
This is beneficial for all parties involved in this sharing of pain.
For one, this will allow us to have one pain token remaining for the Wrack/Haemonculi squad toserve their purpose of being objective holders and whatever you arm these guys with, you will be able to go about doing your mojo; that of having Feel No Pain and get back the pain tokens via nasty killing. The Archon will not only get an huge body of bodyguards to protect its ass, the wyches will also get the two pain tokens it truly need to kick ass; Feel No Pain and Furious Charge. Remember, this happens as a default even if the drugs did not roll a 6 on the drug table.
The second scenario from this is what happens if I somehow managed to roll 6 for the drugs? Well the transfer will still go on, but what changed is that the Wracks/Haemonculi/Archon squad will have 4 pain tokens and the bloodbrides got a single pain token. Now this will give you a lot of options, and you can adapt it to your opponent's army. But in the end, One squad will have Feel No Pain, Furious Charge, and Fearless while the other squad will have both Feel No Pain and Furious Charge. Now I got two killer units that will be there to bring the pain to your opponents.
Of course this pain token thing can also be pulled out with other units, but I find that wyches and Bloodbrides benefit more from this ninja tactic. This is of course a matter of preference, and the unit in which you want to transfer the pain token to is your choice entirely, from Trueborns to Incubi to Kabalites.
In conclusion, pain tokens are quite an interesting and potentially scary way of boosting a Dark Eldar force to terrifying proportions, but that's what the denizens of Commorragh is supposed to do, right?