It's a pretty archaic system really. It works about as well as lifting ones foot off the accelerater. It allows (or perhaps promotes) me to be a bit more ham-fisted with the throttle in low grip situations.

It should be noted that I have a manual transmission. I doubt that an automatic would do this, but TRACS will cycle the brakes to the point where the engine will stall. So there's a delicate balance between applying enough throttle to keep from stalling and not too much that you overpower the system's ability to slow the wheels. So in a very low grip situation like a snow covered hill I sometimes cycle it on to help maintain grip and forward motion then off to keep the engine spinning, then on to get grip again and so forth.

-Will
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850 / Mini