How does HA work? Each ESXi server that is a part of the cluster will maintain its poweron file, which contains the list of VMs running on the host. This file is located on your vmfs volumes in the hidden. vSphere-HA/<FDM cluster ID> directory, as shown in the following screenshot:

The  poweron file is not just used only to track which VMs are running but also for identification if the host is isolated .  The first line of the poweron file contains the isolation identification, and it contain either a 0 (zero) or a 1. A 0 means that the host is not isolated, and a 1 means that it is isolated. The master is responsible for informing vCenter about the isolation status of the host.

When the lock from the heartbeat file is lost, the master ESXi node will check the corresponding poweron list and initiate a restart of the VMs on the other ESXi nodes.