TrueTrac differentials require a certain amount of resistance at the ground (I.e. traction) in order to start the torque transfer. A TrueTrac differential may not transfer torque if the spinning wheel is off the ground or on a very slippery surface. If spinning occurs, often a slight application of the brakes, while carefully applying power, will slow the spinning wheel enough to allow the TrueTrac differential to transfer torque to the other wheel. god bless the internet.