It's a widget and it produces the head. It's nothing actually amazing from a construction point of view. It's just an oddly shaped piece of plastic with a hole in the middle but they spent a lot of money and time coming up with that. The way it works if I remember correctly is that during canning, they inject the drink with a small charge of CO2 which pressurises the can and forces some of the drink inside the widget. When you open the can, you release the pressure and it squirts back out through the hole and produces the head.

The widget is explained here and here. There are other widget types as well.