Let's say you owned the gamertag New York, and you had just bought it from a member on FK.
If you attempted to put that gamertag onto your main xbox account, by changing the GT on that to something random and changing your mains tag to New York, you would be attempting what is called Tag Swapping.
Now, if some random ass kid had a program set on a fresh silver account that checked if "New York" was available every second, and he took your gamertag whilst you were swapping, that is what we call a turbo. Only scumbags turbo though, because it's really just stealing.