High Availability how does it work?

High Availability how does it work?

We have enabled high availability for our ADMP installation and are using a MSSQL database backend on separate servers.  i followed the instructions in the help file on setting up HA so it is configured as directed.  We have now had several instances where the primary server os was updated and restarted but the ADMP service did not start.  When this occurs, the secondary server in the HA did NOT takeover and ADMP was down until the service was restarted on the primary server.

Can we get a description of how HA is supposed to work?  The documentation is minimal in this regard. Does the ADMP service on the secondary need to be running all the time for proper HA function?  How does the secondary server know when the primary is offline so that it takes over?  When the primary server comes back online, does the 'role' switch back or is the Secondary server now the primary, and the primary is now secondary?  
The HA documentation needs to be enhanced to better outline the HA process.  Providing information on how to turn on HA without describing how the HA failover works is not sufficient.