A ping node is a pseudo-member of a cluster. It needs only to be able to receive and reply to ICMP echo requests. Heartbeat places data in the ICMP request that must be preserved in the reply. Since the node is dumb, Heartbeat sends in the echo request what it expects to receive in the echo reply. As long as the replies keep coming, the node is considered to be up.
Ping nodes are currently used by ipfail to diagnose and recover from cluster connectivity failures, and may in the future be used in quorum decisions.