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.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s