The Wiki documenting how to configure IP failover for Hetzner servers  is closely tied to the Linux HA project . This is OK if you want a Heartbeat cluster, but if you want manual failover or an automatic failover from some other form of script then it’s not useful. So I’ll provide the simplest possible documentation.
Below is a sample of shell code to get the current failover settings and change them to point the IP address to a different server. In my tests this takes between 19 and 20 seconds to complete, when the command completes the new server will be active and no IP packets will be lost – but TCP connections will be broken if the servers don’t support shared TCP state.
# username and password for the Hetzner robot
# public IP
# new active server
# get current values
curl -s -u $USERPASS https://robot-ws.your-server.de/failover.yaml/$IP
# change active server
curl -s -u $USERPASS https://robot-ws.your-server.de/failover.yaml/$IP -d active_server_ip=$ACTIVE
Below is the output of the above commands showing the old state and the new state.