It is unfortunate that the HUD does not indicate when Pay Back is active.
The following experiment would verify the behavior of Pay Back: does it proc only after shields are at a full charge, like Nova shields and Brick's Unbreakable skill, or does it "stack upon itself," whatever that means?
Needed: shock sticky grenades, or a weak shock rocket launcher; a slow, low-capacity shield; the Shock Crystal Cavern with no hostile fauna currently in the vicinity.
Punch a shock crystal cluster, and observe the damage number. Strip your shields by throwing a shock sticky grenade at your feet, then punch the crystal cluster a second time; observe the new number. Wait 15 seconds (long enough for the original Pay Back effect to dissipate); at any time in those 15 seconds, if your shield regenerates above 50%, throw another shock sticky at your feet to strip it again. After 15 seconds, punch the crystal cluster a third time, observing this final number.
If Pay Back only activates from a full shield charge, the third number will be the same as the first.
Goetter 02:32, February 9, 2011 (UTC)
Confirmed not working[]
Tested it at all 5 ranks with the method listed above, there was no change in damage.
To clarify: Payback doesn't seem to activate at all. I was hitting for 206 damage with before and after fully draining my shields, as well as draining from half to zero, and at all 5 ranks.
A simpler test[]
I took a simpler approach to the proposed testing method, since I am too lazy to round up shock grenades etc. This is on the PC, patch 1.41.
Initially, I took a Berserker with 5/5 Pay Back and 5/5 Unbreakable just outside Fyrestone. Cleared the area of all skags and 2 out of 3 bandits. Stepped into cover to let all on-kill buffs subside. With no buffs active, took a single shot with a non-elemental repeater at the sole remaining bandit: 508 damage. Stoically remained in place until fire from the bandit stripped all shields. Immediately took a second shot: 508 damage. Incidentally, I did not see Unbreakable proc.
In disbelief (as I initially interpreted this as Pay Back is broken!), I then took the character to the yard behind Lucky's and let him get beat down by a couple of bullies. This time, I did see Pay Back activate: shots that inflicted 350 went to 490, exactly as a 40% damage buff should do. What's more, I saw a shield-level bounce from Unbreakable activating that I didn't see in the first test. I can only infer that in the first test, I didn't let the shield regenerate completely.
Took the character back to Fyrestone, let the fauna there respawn, and tested again: take a shot with no on-kill buffs active, let enemies strip shields, take another shot at the same enemy, compare numbers. As at Lucky's, I again saw Unbreakable trigger, and again a damage increase.
From these results, I infer that Pay Back has the same preconditions as Unbreakable. If Unbreakable does not proc, neither will Pay Back. Kind of sloppy tests, but interesting nevertheless. Daemmerung 20:34, February 25, 2011 (UTC)
Pay Back definitely works on enemies. I have not tried it on crystals. 
20:36, February 25, 2011 (UTC)
- So I've found. I jumped to erroneous conclusions from a flawed initial test. Sorry for the ninja edits. Daemmerung 20:38, February 25, 2011 (UTC)
Not a problem. Testing is the best way to get to the bottom of things. The notes section has been updated. 21:14, February 25, 2011 (UTC)
- My testing backs the hypothesis that Pay Back shares its trigger preconditons with Unbreakable. What's the procedure for clearing a "citation needed" tag? (What does "citation needed" even mean on this Wiki?) Daemmerung 21:19, February 25, 2011 (UTC)
Citation is back up confirmation with a source. I have no idea on procedure. Start a forum page suggesting it? :) 21:32, February 25, 2011 (UTC)
- NOhara sez "just do it," so I dood it. Daemmerung 23:11, February 25, 2011 (UTC)
One year later: my crystal test was flawed because crystal attack numbers do not reflect any buffs. See Talk:Melee#Damage. Dämmerung 21:57, January 16, 2012 (UTC)
Further tested with a Warmonger[]
Only Brick's melee attacks made while Berserk receive a bonus from Pay Back. Strikes made with a bladed weapon such as Sledge's Shotgun or with the default club-- that is, melee attacks made while not Berserk-- ignore Pay Back.
Easiest to observe this with no artifact selected for Brick's punch, so as not to obscure damage numbers. Dämmerung 16:27, January 26, 2012 (UTC)
P.S. I still need to test whether Pay Back affects rocket launcher damage (which Revenge doesn't) or grenade explosive damage. Dämmerung 16:40, January 26, 2012 (UTC)
Confirmed: like Revenge, does not affect RL damage output. Dämmerung 20:34, January 29, 2012 (UTC)
I tested the Rocket Launcher thing myself, using Rakk Hive as the target. If definitely DOES apply to rocket launchers (as of latest patch). The increase was equal to 7% per skillpoint, just as I had observed in the Unreal Editor (the var is called GrenadeDamage but it affects Rocket Launchers too, and that var does not appear in the Unreal Editor under Revenge. The Unreal Editor shows that Payback gives the following per skill point:
+8% bullet damage (bullets only)
+7% grenade damage (also affects rocket launchers)
+7% melee damage (should apply to all melee attacks, whether or not Brick is using Berserk)
+7% status effect damage (elemental DoTs)
Frightning Lightning (talk) 08:40, July 18, 2014 (UTC)