The metal plate worn by Dorje Shugden oracle is not unique, but something worn by most oracles when they take trance. For example, the Nechung oracle uses this metal plate as well. This metal plate is known as thungs kyi me long aka divination mirror. The tradition of this mirror comes from centuries ago, even as far back as pre-Buddhist era. During that time, mirrors do not exist, and to see ones reflection, metal is used… thus the metal plate functions as a mirror. Also, there are many other ancient religions that use mirror as ritual objects, especially for divination purposes. At the center of the plate, the deity’s seed syllable is inscribe, example HRI.
As for the function of the mirror, I am not 100% sure, however, it is logical to look at it as
1) Protection. The mirror which is inscribed with the deity’s seed syllable protects us from the wrong entity from entering the oracle.
2) Since it is also known as a divination mirror, it helps the deity to divine the future when we ask for advice and help.
As for the oracle in trance hitting the metal plate, it is usually when the Lama requests for his assistance, if the deity wishes to help, the hitting the metal plate is a way of communicating he promises to assist.
Hope this helps.
