You can find the location below:
-High-Availability-Support-in-7.1.1-Firmware-kA1VN0000000ODk0AM-0EMVN00000EoOXK.png)
When there are three consecutive failures in communicating with the CSa that is currently being used (whether original or alternate), the UTM will automatically failover to using the other CSa. Failures in file transfers as well as connectivity tests are counted for this purpose. An event log (msg-id: 1689) at NOTICE level will be generated for any failovers.
There is also a button called, "Initiate" Beside āLocal Capture ATP Force Failoverā, that can used to force this failover from one CSa to the other manually when desired.