I believe he is referring to an event such as a javascript based event when a user clicks a button etc. I'm also interested in event tracking because my WebGL site uses it to track user interaction since users only visit the WebGL view and don't visit multiple pages.

Piwik calls those "goals". You can definitely kick those off from javascript. We run piwik on our web solitaire site (http://greenfelt.net) and kick off goals when users do things like complete a game. The code looks like this:

    if (piwikTracker)
        piwikTracker.trackGoal(2);

My only annoyance is that you have to reference things by numbers and not some nice string that reads well in the code... But it works and it's something you setup once and forget about, so I got over it.