The Tracker uses event handlers to determine whether the user is browsing actively. You can see a list of the listeners here.
snowplow_name_here('enableActivityTracking', 30, 10);
If the Tracker is configured this way, it will first wait for 30 seconds before sending any page pings. After that, every 10 seconds it will decide whether to fire a page ping event based on whether any of those activity handlers have been triggered.