There is a messaging bus in ServiceNow, Asynchronous Message Bus (AMB) which enables real time communication between the instance and the other related components.
One such is the interaction is with the client-side component; when something is updated, AMB gets hold of it and updates the client, which internally triggers the logic to show that heartbeat icon on the updated fields on the form
What's the underlying technology, you ask?
Well AMB is based on Web Socket technology that enabled real-time 2-way communication, which is the same that is used for chats and other real-time applications!
