After you complete your EPA, you will first need to sign and witness the document with a registered Hong Kong doctor and certified lawyer. This is a legal requirement to verify that you are mentally capable of making such a decision. (After signing the documents, you should also request from them a certified copy of their practising certificates to be kept with the EPA, in case the high court requests for one when the attorney registers the document.) The next step is for your chosen attorney to sign and witness the document, acknowledging their role.
If you unfortunately loose mental capacity, then your attorney will need to bring this document to the High Court and register the EPA. Once approved by the court, they will be able to access your financial assets.
In the EPA, you can opt to require the attorney to notify a third person before they bring the document for registration.