ADFS 2.0 SQL DB failover - how does it work?

I have configured an ADFS 2.0 farm with three web servers, two in the main site and one in our DR  site.  I will be using an f5 to do network load balancing across the three web servers, and for the Databases we have two SQL servers that our DBA's set up in a mirror config,  and the three web servers have this configured as their connection string for ADFS Configuration:

"Data Source=aADFSDB;Failover Partner=aADFSDBDR;Initial Catalog=AdfsConfiguration;Integrated Security=True"

and the artifact store DB connection string:

"Data Source=aADFSDB;Failover Partner=aADFSDBDR;Initial Catalog=AdfsArtifactStore;Integrated Security=True"

so as you can see SQL Server 1 (aADFSDB) hosts the active Database and SQL server 2 (aADFSDBDR) hosts the failover partner/mirror.

How exactly do I get ADFS to failover to the second DB server?  I tried blocking one of the Web server's access to the main database server hoping I would see an event confirming it attempting to connect to the failover partner but all I got were event's like the one at the bottom of this page.

So can anyone tell me how exactly this configuration fails over to the failover partner DB, what is required for it to do so and how I can test it?  I have looked for documentation but Microsoft give precious little info on using SQL, which we needed for security reasons (token replay detection is only available in SQL config not windows internal DB config)

And I found many pages on how to setup the SQL with failover partner config but none with any details on how ADFs actually detects and performs failover and what manual steps may need to be taken in the event that it does.