So kind of like driving up a steep grade with an open diff, you click the parking brake down a few notches?

 

yeah, something like that.

true tracs, or any heilical type LS (torsen), bias torque through a set of gears which gives it a mechanical torque multiplying effect.

for the sake of arguement, let's use 4

so, if one wheel can sustain 50lb-ft of torque, but starts to slip, the internal gears will transfer torque to the other side and in doing so multiplies it by 4. so up to 200 lb-ft of torque can be transfered to the other side, until that one breaks lose too.

but on ice, you may only be able to sustain ... say ... 4 ft-lb of torque ... so there is only 8ft-lb available to the other side. may not be enough for forward momentum, and you just spin one tire.

click the parking brake down a few notces puts resistance to the wheel on ice, now say 20ft-lb. so there is 80ft-lb available to the other side, but there is a brake applied there too, so it turns out to be 80-20, or 60ft-lb available for forward movement .. that might get it done.

this works until you burn the brakes up.

when the 1wsc kicks in, you are only applying to the wheel spining, so that could work out very well overall.

to get moving on an icy hill, i could amost assure you would need some brake action to get the other side to have enough torque to move the vehicle ... but other than that, the gear type seem like the way to go.