In normal conditions the drive is split 50/50 fronto to back.
The VCU locks up when the axles are turning at different speeds to eachother (due to wheel slippage/spin) the lock up ensures that the drive is sent to both axle equally rather than sending all the power to the spinning axle and losing traction.
The Freelanders VCU splits the power mostly to the front wheels, and in the event of front wheel slip, the VCU locks up and send equal power to front and rear.
With the Front Prop disconnected, you are emulating front axle slip/spinning, so the VCU locks up and ensures the rear axle is driven at the same speed as the 'Front'