Hypergems always occurred in matched pairs. Low-energy wide-band transmissions through hyperspace could be made between them, with no added power or equipment.
The Shenn used them to control the duplicates they created. It allowed instantaneous and continuous control between the original and the copy, with the Shenn inside able to override the link when necessary. They could thus send them from Arden to Tairngaire without any reduction in connection. (PROSE: Shadowmind)