are you talking about tail blocking? Some QH/reining people (I think it's mainly QH and reining) inject something into their horses' tails so that can't move them at all or very little, making it appear that your horse is calm and happy, thus not swishing its tail! I worked with a mare who had her tail permenantly blocked and I didn't even realize it until the vet was messing with her tail. He ticked her under her dock and normally horses will move their tails around when you do that. She just kind of barely moved hers over to the side. There aren't any visable signs on her, other then having a limp tail!
I don't think it's technically considered abuse (correct me if I'm wrong on this!) but it's not good (they need their tails!) so if I remember correctly some breed organizations will deduct points or disqualify you if your horse's tail is blocked.