The subordinate checks the health of the Master by sending heartbeats to the Master. Technically these health checks are requests to the Master for its current state.

The heartbeat is a request that is sent through a UDP socket. When the Master sees the heartbeat, it responds to the sender with its current state. The Master also sends heartbeats to the subordinate DNCA host machine to keep track of its state to determine whether it can failover to the subordinate.

Heartbeat Interval
How long to wait between heartbeats
Heartbeat Timeout
The amount of time DNCA waits for a response to a heartbeat before you call it a timeout
Timeout Limit
The number of consecutive heartbeat timeouts that are allowed before a failover is forced