The heartbeat datastore is used to know if the ESXi is down or isolated in case of failed of the management network. By default the datastores are automatically choosed by the vCenter (2) and a folder .vSsphere-HA is created/used for heartbeat and listing of VM protected by HA.
You can configure what do you want to do after an isolation via HA response types :
Leave powered on– When a network isolation occurs on the host, the state of the virtual machines remain unchanged and the virtual machines on the isolated host continue to run even if the host can no longer communicate with other hosts in the cluster. This setting also reduces the chances of a false positive. A false positive in this case is an isolated heartbeat network, but a non-isolated virtual machine network and a non-isolated iSCSI/NFS network. Should the host become unresponsive or fail and can no longer access/run the virtual machines, the virtual machines will be registered and powered on by another running host in the cluster. By default, the isolated host leaves its virtual machines powered on.
Power off– When a network isolation occurs, all virtual machines are powered off and restarted on another ESXi host. It is a hard stop. A power off response is initiated on the fourteenth second and a restart is initiated on the fifteenth second.
Shut down– When a network isolation occurs, all virtual machines running on that host are shut down via VMware Tools and restarted on another ESXi host. If this is not successful within 5 minutes, a power off response type is executed