Specifically, it is from a bottle of Guiness Draught, regular Guiness Stout doesn't use it. In Britain and thereabouts, draught Guiness is much different than the regular bottled stuff you get here. It has a ton of foam that is carefully controlled by a "pour" that takes a least a couple minutes and sometimes longer to get it "just right".

A bartender that pours one perfectly is said to have built a good Guinness and the foam head will often have a perfect shamrock "drawn" in it by a fancy twist of the wrist at the end of the pour.

Anyway, the rocket widget is there to both provide and control the foam head in Guiness Draught bottles.

There...more than you ever wanted to know about the subject...it has been a pleasure educating ya'all.

I wish I could see the European methods firsthand... but thanks for the field trip, lash

well I can viuch that the widget I took out of the guiness I had in Quatar was a little round ball

the balls are in the cans, the rockets are in the bottles