How does DataContractJsonSerializer.ReadObject(XmlReader, Boolean) read json from an XmlReader?

Question

I'm looking at the ms documentation for DataContractJsonSerializer which lists an overload for ReadObject:

ReadObject(XmlReader, Boolean)

The documentation states:

Reads an XML document mapped from JSON with an XmlReader and returns the deserialized object; it also enables you to specify whether the serializer should verify that it is positioned on an appropriate element before attempting to deserialize.

What does this mean? how do you get a json object into an XmlReader?