This is an accumulator
Actually a slow speed one
I have worked on machines with accumulators with rolls that are 72” wide and weight about 10 tons
https://www.youtube.com/watch?v=9rErS94Tl8M
https://www.youtube.com/watch?v=KJJ8LUKl5zs
there are many other examples of accumulators out there
on some of the high speed web presses the accumulator holds about 500 ft or more of paper to allow for roll change with shutting down the press run over 500 ft / min
I have never run into a dancer that acts like an accumulator. Dancer rolls are never powered they are a feedback for tension on the web. Dancer position or the newer one are mounted on load cells.
The example shown is a drop loop and yes some consider it an accumulator but it only works at low speed with a small loop of the web. A drop loop can never be used for tension it should be obvious here is no tension at all
If you look closely you will see most of the unwinders are using a brake on the unwind roll to maintain tension on the web newer machines use a VFD running in regeneration mode to control tension.
To control you need something to pull the web, something to either hold back the web(Break, Drive running in Regen) and some mechanism to measure and feed back the tension
I will say it again we are not getting the full picture the OP may not know the full picture. But without the information nobody can point you in the right direction.