Question about Guinness draught

[Guest Incandenza]

What's with the so-called "rocket widget"? It's what they call that little plastic thing in the bottles of Guinness. I went to their website, but found nothing. (I'll admit I didn't look vey hard.)

 

EDIT: Hey, I found my answer elsewhere. Cecil Adams of Straight Dope fame supplies the answer. The material's dated (bottles didn't have the plastic thing at the time it was written), but it gives me what I'm looking for.