To achieve a torque split you monkey with gear ratios.
On the Cosworth centre diff, there is what is known as an "epicyclic gearset".

Like this:




The centre gear is called the "sun", the three gears "planets", and the outer ring the "annulus".

On the cossie, power is fed to the planet set, rear drive is taken by the annulus, front drive by the sun.

The viscous coupling links sun and annulus to promote an equal rotation speed.

The relationship between ratios (planet gear to sun and planet gear to annulus) decides your torque split.
There is obviously a big difference between planet to sun ratio, and planet to annulus ratio, and when you consider that every bit of gear reduction equals torque multiplication, you can see that there will be a greater torque multiplication to the annulus than to the sun.