Mr. Eppingâ€™s High Availability â€œDeepdiveâ€ page is definitely worth a read and a book mark.
A VMware HA Cluster consists of nodes, primary and secondary nodes. Primary nodes hold cluster settings and all â€œnode statesâ€ which are synced between primaries. Secondary nodes send their state info(resource occupation) to the primary nodes
Nodes send a heartbeat to each other, which is the mechanism to detect possible outages. Primary nodes send heartbeats to primary nodes only. Secondary nodes also send their heartbeats only to primary nodes. Nodes send out these heartbeats every second by default. However this is a changeable value: das.failuredetectioninterval. (Advanced Settings on your HA-Cluster)
Lots of good stuff in there.