Blame Jerra.

       Alternate title?  - SO!  That's HOW they do it??  
Why blame him?  Well it befell thusly:-
      He was recently forced to resort to breaking & entering on a recalcitrant Can of "Arth Guinnefs" finest.  He then asked me if I'd ever seen a Guinness Widget and wondered how they got the gas in.
      Being both A) ME  &  B) "Iggorant" I had to find out.  If I'd known it was Nitrogen I might have guessed.
        Though I've no doubt all such follows a similar system I found that:-
"During the canning process, brewers add liquid nitrogen to the beer immediately before sealing it up.  The liquid nitrogen quickly evaporates in the container.  But since the can is sealed, this creates pressure inside it, which then forces gas and liquid into the widget through its tiny hole, per Guinness. 
Then, when you open the can, you should hear a "pssshhh" sound.  That's the gas and liquid leaving the widget, which mixes with the beer, creating that foamy head so iconic in a Guinness draft, Guinness said."
        I found two tiny indents, (One. circled) one of them has teeny weeny hole a guesstimate fraction of a mm in diameter.
       A bit more guessimetry - BUT - since the Can is fairly pressurised the bulk of the nitrogen stays within the widget until the pressure outside it is lowered by opening; hence the PSSssst!