Chapter 1 What’s New for FortiOS 5.2.1 : High Availability : Trigger Failover
  
Trigger Failover
HA failover can now be enabled and disabled using the following CLI commands:
diagnose sys ha set-as-master enable: immediately enables the local FortiGate unit as the HA master.
diagnose sys ha set-as-master disable: immediately disables this mode. Optionally, a time frame can be added after disable, which will disable the mode at the appointed time. The time format is yyyy-mm-dd hh:mm:ss.