Virtual machines (VMs) must enable encryption for Fault Tolerance.
Fault Tolerance log traffic can be encrypted. This could contain sensitive data from the protected machine's memory or CPU instructions.
vSphere Fault Tolerance performs frequent checks between a primary VM and secondary VM so the secondary VM can quickly resume from the last successful checkpoint. The checkpoint contains the VM state that has been modified since the previous checkpoint.
When Fault Tolerance is turned on, FT encryption is set to "Opportunistic" by default, which means it enables encryption only if both the primary and secondary host are capable of encryption.