heart beat is a signal,this signals are sent by objects.

think that there are 5 servers.(the servers with in the 
cluster to communicate each other they use multicast ip)
if u add one objet to any  oneof these server that object 
will send a heart beat signal to multicast ip from there 
remainig servers will receive that signals and update thire 
jndi tree.from this only each server will known the helth 
of each object.if the object is stop sending heart beats 
the servers treat that that object is failed

Yeah above answer is also right..
I would say like this.
There are 5 managed servers in a cluster.Then each managed 
server will communicate each other with the help of 
mutlicast ip.After every 3 sec heartbeat checks whethe 
anothers are alive or not.If it is alive it says I am 
alive.or else no response.Then the conclusion is that 
perticular managed server is not alive.

Heart Beat messages send every 10 seconds not every 3 sec.
If 3 consecutive attempts will fail, server marked as a failed.
Let me know if any conserns.