In the traditional Punch-and-Judy show, Mr Punch speaks with a high squeaky, rasping voice, interspersed with ear-splitting cries when he perpetrates some piece of devilry or is thwarted. The showman makes these noises by means of a device in his mouth, these days usually called a swazzle.
This has taken various forms but in essence is a small pair of bowed plates with tape across the middle. The Punch-and-Judy man holds it near the back of his mouth and blows through it, much as a clarinet player does with the reeds in his instrument. It takes a lot of practice to make recognisable words and to swap between Punch’s swazzled rasp and the unswazzled voices of the other characters.
Swazzle is a modified form of the older swatchel; this might be a variant of swatch but is more probably from German schwätzeln, taken in turn from schwatzen, to chatter or tattle. One reason that experts think this is the source is that the very earliest example, dated 1854, spells the word schwassle. In the latter part of that century, the swatchel-cove was the Punch-and-Judy man or his assistant who did the supporting patter and who interpreted Punch’s less intelligible squawks, and the swatchel-box was the booth in which the Punchman stood.
Incidentally, there are some risks attached to using the swazzle: tradition requires that the performer shall swallow it at least twice before he can call himself a Punch-and-Judy man.