The 'widget' has evolved appreciably over the years. The last one I looked at was about the size of a ping pong ball with a very small hole at one pole. These are, I assume, filled with nitrogen at atmospheric pressure before being dropped into the can. The can is then filled with beer and a little liquid N2 is squirted in the instant before the lid is clamped on. The can then goes into the pasteurizing tunnel where the heat causes the N2 to evaporate and the pressure becomes quite high so that the beer, at this higher pressure, is forced into the ball. The can is then sent off to the store, put in your refrigerator and eventually taken out. Even though it is now cold the partial pressure of nitrogen in there is a couple of atmospheres. When you open the can the pressure in the head space drops instantly to 1 atm but the pressure in the ball is still much higher than that as it can only bleed off through the tiny hole. In trying to equalize the pressure nitrogen and beer are forced out through the tiny hole. The widget is designed to spin as a result of this jet of beer. This jet of beer agitates the main volume of beer thus causing the CO2 in solution to come out in much the same way it does when the beer is agitated by being forced through a sparkler or restrictor plate.