Standard Play/Pause button
There's no mystery on how the Play/Pause addon button works, the only thing to consider is that, as interactions it has its duration, the option to upload your own images instead of the default ones, transitions and on mouse over, or hover behaviour.
Full screen behaviour
There is an option on the General submenu of the Properties panel making the Play/Pause button into an invisible controller in full-screen mode. You can activate this option by enabling the Full screen selection box. If you select it, button designs are no more effective and the whole canvas of the player will behave as a big Play/Pause button.