This event is aimed at measuring how much time users are actually spending engaged with your content. The idea of this event is based on the research done by Chartbeat. The implementation is also covered by Simo Ahava. 

According to Chartbeat’s research, after each interaction the user usually remains engaged for 5 additional seconds. What this measurement does, is that it listens to user interactions like moving the mouse, clicking, scrolling or pressing a key and starts a timer that tracks 5 seconds of engagement. When the user performs another action, then 5 additional seconds of engagement is added. Every 15 seconds of cumulative engagement is submitted as an event.

Parameters

engagement_time: time in milliseconds for which the user was engaged with your content

Utilization

Engagement time is an interesting metric that gives you the actual engagement time of users while they have the page opened. This goes beyond time on page which simply tells you the time that has passed between the opening of 2 pages, but does not take user activity into account. You may use this metric to create audiences which have been deeply engaged with your content to uncover differences in how they use your websites or to deliver targeted advertising to them.