I know the actual result of the Widget, a fine, rich froth. I know the basic mechanism, pressurized gas escaping when the whole can depressurizes, and pushing the beer out from the widget. I still don't understand how this whole "activation" process works.

What physical processes occur in the liquid that result in this particular effect. as opposed to, say, big, coarse big bubbles as would blowing air through a straw into beer create?