What will be the effect of disconnecting the power cable of a trailer with two-circuit pneumatic braking system from a tractor?

In a dual-circuit pneumatic braking system, the air pressure supplied from the tractor unit maintains the trailer's brakes in a released state. Disconnecting the supply line causes a drop in pressure in the system, which results in the automatic, emergency application of the brakes. This is a deliberate safety feature which prevents the disconnected trailer from moving.