Wyrmsign Rampage mechanics

So im trying out Dancing Duo build.
so i though why should i use 3 writhing jar when i can just use 1 doedre's Elixir to gain 3 endurance charges so i can enter rampage when i get maximum endurance charges.
the problem happened when i got maximum endurance charges with flask/enduring cry/ fragility(x) and minimum endurance charge craft on ring, but rampage never starts and doesn't work in either ways.
Is this item doesn't work or is there missing text that i can't see?
i tried with from 5 to 1 max endurance charges and they never worked.
Help

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After getting maximum endurance charges you have to kill multiple mobs in short order, then you'll get the rampage counter.

I just figured it out how they work.

"Gain Rampage while at maximum endurance charges" should be renamed.
Cause "triggers level 15 Manifest Dervish on Rampage" should work from description of wyrmsign.
Or dancing dervish/duo "triggers level 15 Manifest Dervish on Rampage" should be renamed to "triggers level 15 Manifest Dervish on Rampage counter" or something like that.