Looking at a twisted-pair, shielded, balanced T1 cable, any signal from outside of the shield is noise. The shield serves as an antenna for electromagnetic (EM) energy. This antenna receives EM energy from the air that induces a voltage in the shield which, in turn, has the potential to cause a current flow in the shield. This current is carried (drained off) by the grounded shield, which is where the "draining off noise" term comes in. The shield serves as the EM boundary or guard for the twisted-pair transmission line. The quality of the shield is determined by its ability to convert the incident EM fields to electric currents.