I "pop" Cin's tail all the time but I've never heard it called that. My chiro suggested it. You gently grab the tail hairs and pull back slowly and evenly. Cinny likes it so much that he actually raises his back leans against it. My chiro says it stretches the whole spine, not just the tail.
I would be careful with MTG, some horses have a reaction to it that could make the tail rubbing worse. Have you separated the hair and looked all the way down at the skin for flakes, rawness, blisters. etc. I would take a good look at what the skin looks like under there. She may have fungus, an allergic reaction to something etc. "Eqyss" makes some good products that help in a lot of different situations. Eqyss Products
Also when bathing triple rinse the tail bone. Sometimes you think you have all the soap out and you don't. I know it sounds elementary, but I still have on occasion found that I haven't gotten all the soap off of Cin's tail even when I thought I did.