Lallypop, I don't know if it really means anything when they hold down their tails except they are keeping them in check. This is solely my own observation. I've watched Briggs when she's curled up tight, ready to nap and has her paws on top of her tail. Briggs will be half asleep but her tail is wiggling under her paw as if it has a mind of it's own. I really think she's trying to keep it from escaping and waving around while she's resting.