Seems pretty clear to me, Roy. The effectiveness of a copper strap
around a mains-frequency power transformer at reducing the exterior
magnetic field is well known and often used. It's all very clear from
Faraday's law of magnetic induction: the net magnetic flux through an
area enclosed by a perfect conductor may not change, so time-varying
magnetic fields are perfectly blocked by perfect conductors. Copper's
a reasonable approximation of a perfect conductor in the case of RF
shields.

Cheers,
Tom