Only if one wheel is spinning faster than the other. If both are identical, then you can sit there and spin the rears all day long.
It should come on as soon as you get the arse out to any great degree though, so there's something not quite right there. Without actually trying it (unless you've got some private land), do you think you could spin the car with enough throttle when turning? Mine allows a small amount of slip in this weather as there's simply not enough grip even if the TCS kicks in straight away, but it kills the power long before I get get into any real trouble.