Since JIRA Agile 6.0.4, the boards will automatically poll for changes to the board every 30 seconds as long as the browser is in focus. Any new changes will then be reflected on the board.
Reference: https://jira.atlassian.com/browse/GHS-5568

The polling period should be a configurable value (or disabled entirely), as it helps customers maintain control over the performance of their instance. (preventing large boards from polling to help reduce load on the server)