The badge is earned for giving quality answers, and for contributing a lot of answers, and also for getting above a certain % percentage amount of best answers during each particular week. Badges are awarded or taken away usually early each Monday.
There are only rumors out for us, about how many questions, or how high a percentage % of best answers we need to get to keep our badges each week. I myself usually answer at least about 10 or more questions in my TC category per-day. I try to be as informative as I can, to help folks in my badge category. I am unemployed so I have had time to keep up with things, when I finally get back to work, my T.C. Badge will probably go bye bye too, as I don't think I will have enough time to answer as many questions.
I typically get just under or just over about 1000 points per week, which are a combination of the number of answers and the number of best answers, I don't worry about the points so much, as I just enjoy giving others helpful information, mostly about ipods and such as I can. It's fun for me when I have the time, I really don't care all that much if I have the badge or not, although it's nice to have it, so it makes my answer stand out a bit more, because of an orange blotch under my avatar.
You can get the badge earned back again, if you start answering enough questions and get some best answers again :)