FT creates a secondary VM on another ESX host that shares the same virtual disk file as the primary VM. Then it transfers the CPU and virtual device input from the primary VM (record) to the secondary VM (replay) via an FT logging NIC so that it is in sync with the primary and ready to take over in case there's a failure.
Data from the primary VM is copied to the secondary VM using a special FT logging network that is configured on each ESXi Server.
VMware has a formula that you can use to determine the FT logging bandwidth requirements: VMware FT logging bandwidth = (Avg disk reads (MB/s) × 8 + Avg network input (Mbps)) × 1.2 [20% headroom].