Traction control works (in general) by sensing the level of grip at each corner, the theory being that if one corner loses grip then you're either driving like a tit or you're driving on something you shouldn't be, so it'll remove power from that corner to sort things out. I think the Zed works by simply cutting spark to the plugs so you get zero power to the back end.
With brand-new and ultra-slippy (for the first thousand miles until the release agent has scrubbed off) Toyos on the back and Bridgestones on the front, worn or not, everytime you put your foot down even a fraction the rear is wanting to step out more than the front, whether you can feel it or not. The car reacts quicker than you, so kicks the TC in to sort you out.