HOTSPOT
You have an Exchange Server organization that contains the servers configured as shown in the
following table.
You need to configure the namespace to ensure that users can connect to Outlook Web Access from
the Internet if a single site is unavailable.
What command should you run on EX1 and EX2? To answer, select the appropriate options in the
answer area.
Answer: 
Explanation:
Set-OWAVirtualDirectory -FailbackURLSet-OwaVirtualDirectory
https://technet.microsoft.com/en-us/library/bb123515(v=exchg.150).aspx
When we are using outlook anywhere we would configure the external URL to be the same as the internal URL for the other site.
However, with OWA, this would require the user to manually log out and back in (and would require them to know they need to go to the alternative URL).
With Failover it will manually do this for them.
This is how I guess it would happen anyway…
http://blogs.technet.com/b/exchange/archive/2011/12/12/owa-cross-site-silent-redirection-in-exchange-2010-sp2.aspx
Ignore my comment above, I think it is externalURL, failback is once you have restored the site after a failure.
The FailbackUrl parameter specifies the host name Outlook on the web uses to connect to the server after failback in a site resilience process and requires a separate DNS entry pointing to the original server’s IP address. The FailbackUrl parameter must be different from the ExternalUrl parameter.
The ExternalURL parameter specifies the URL that’s used to connect to the virtual directory from outside the firewall.
The FailbackURL looks like an OWA ExternalURL, but uses a different hostname and never has its DNS entry changed as part of any kidn of datacenter switchover event.
https://social.technet.microsoft.com/Forums/exchange/en-US/91ce2960-1e3f-4716-a173-c15f1b32fc90/failbackurl?forum=exchange2010