ServerBeat is a special protocol that will dynamically assign the specified IP between multiple CrushFTP servers.  You specify an IP that is free.  You then set any NAT port mapping or DNS records to point to that IP.  The CrushFTP instances that are running configured for that virtual IP via ServerBeat will share the IP.  Only one machine will have the IP at any time, and it will hold onto it unless its powered off.  In that case, within just a few seconds, the other CrushFTP server will take over and start using the IP.

This is ideally suited for two machines in a failover scenario, but it could be used on 3 or 4 if you really wanted to.  Still only one machine is active at any time.  When Crush changes the IP, its changing it in the OS.  Technically its assigning an IP alias for the machine.

ServerBeat has some advanced options for configuring what ethernet Adapter to use, and what virtual IPs to use.

These settings are on the advanced tab of the port.