In the last article: http://www.evwaudi.com/2011/02/how-to-safely-work-vw-touareg-hybrid-disconnect-battery-pack/ the main safety switch was described.

This article will describe how the safety line works to disconnect the electrical system on the VW Touareg hybrid and Porsche Cayenne S hybrid.

Safety line closed:

The VW Touareg hybrid’s high voltage elements are linked by a separate low voltage loop line. Each connection with the safety line is designed to act like a contact breaker point.  When all components are ready to operate, the contact breaker points are closed.  When the safety line sees voltage, current is able to flow because the line is not interrupted. The presence of current indicates that all of the safety line’s components are ready to operate. The function of the safety line is comparable with cold monitoring in the case of bulbs.

Safety line open:

If any point is open the safety line is interrupted.  This could be if any component is not ready to operate or because the safety connector has been removed.  In this case, there will be no current. This indicates that the high voltage system is not ready to operate. The check to determine whether the safety line is closed or interrupted is carried out by the voltage converter. If the control unit determines that the line is interrupted, it actuates the protective relays and interrupts the high voltage battery’s connection to the high voltage system.

Below is an illustration from VW’s literature showing the safety line.