Fallback services are automatically applied to all leases on a first in basis, aka the first nodes to go down will take whatever fallback IPs are available. There is enough IPs for most nodes to have a full downtime and for all customers to receive a fallback IP.
In order for this to work you must be setting up your domains using the CNAME alias provided.
During the time in which the proxy is down all requests will hit your server with a different IP if you have a firewall please take this into consideration.
Ensure you do not have a firewall if you wish the fallback service to work. Due to the nature of the IP selection you will be unlikely to get a predictable IP address. IP addresses are picked from a pool according to an internal ranking algorithm.
This feature only applies to Reverse Proxy services, it does not apply to DDoS protected services utilizing a reverse proxy backend method (Ports containing "RP"). Multihomed DDoS protected services offer a more advanced solution to this problem.