To simulate a failure of region A, we'll simply stop the web server, which is attached to the ELB as follows:
[root@ip-172-30-0-153 ec2-user]# service nginx stop
Stopping nginx: [ OK ]
After a short period, the web console will show that the health check for region A is failing:
Region A failing health check
Our North American user (left) now sees region B instead:
Left: Region B served to North American users due to failover. Right: Region B served to Australian users as normal.