Overview
A widget is a device placed in a container of beer to manage the characteristics of the beer's head. The original widget was patented in Ireland by Guinness. The "floating widget" is found in cans of beer as a hollow plastic sphere, approximately in diameter (similar in appearance to a table tennis ball, but smaller) with two small holes and a seam. The "rocket widget" is found in bottles, in length with the small hole at the bottom.
Background
Draught Guinness, as it is known today, was first produced in 1759. With Guinness keen to produce draught beer packaged for consumers to drink at home, Bottled Draught Guinness was formulated in 1978 and launched into the Irish market in 1979. It was never actively marketed internationally as it required an "initiator" device, which looked rather like a syringe, to make it work.
Method
From Wikipedia (CC BY-SA 4.0).