You don't need any special supersecret spec. They're using a simple trick by making themselves surrounded by low level mobs and then triggering Thirst for Blood, wait for the stacks to reach 10+ (you can actually go to 15 if you're in a full group), then trigger Too Many to Count (full 10 stacks) plus whatever additional damage buff you'd like (Haste mainly, but you can also use e.g. Blood Fury for the damage increase) before hitting your pal with the superbuffed Decap/AtG. Bonus cheat points if you do this with T4 PvE gear and then switch to PvP one for the screenshot, although it's not necessary. The total weapon damage bonus will be utterly insane, and thus it will boost the Decap/AtG damage to absurd levels.
Of course, reaching 10+ stacks of Thirst for Blood and Too Many to Count and survive (you need 10+ enemies around you for the Too Many to Count, and barb doesn't have bubbles) is next to impossible in any realistic pvp situation; not to mention that those abilities are on a 90 and 180 seconds CD so it's not like you can use them on every Decap... or even the majority of them; but that obviously doesn't stop them from claiming you can "often" hit for 5k+ with barb abilities.
This is actually pretty conclusive evidence that there's a few people around here whose "feedback" is simply based on their own personal vendetta against the barb class. No one in his right mind who's trying to provide real feedback would try to seriously claim you can easily hit for 4-5k on both Decap and AtG (basically one shotting all non-tank targets in your frontal cone every 30 seconds) since if this was actually true the whining in every single pvp subforum would have reached the sky by now, not to mention you'd find 6+ barbs in every single mini, you'd see barbs utterly dominating the first 5-6 spots in any JS etc. etc.