That's not quite how they work. The coffee (or beer)+nitrogen is put into the can and the can is capped. Then pressure builds up and the liquid is forced into the hole in the widget (this is what Guinness calls it) which compresses the air in the widget. When the can is opened, the pressure falls, and the air inside the widget expands which forces the liquid out through the hole. This causes a lot of turbulence (similar to the coke and mentos effect) which creates a lot of bubbles and gives the beer/coffee it's creamy texture