From Discworld & Terry Pratchett Wiki
A shamble (also called a shambles) is a handmade device used by witches to detect or amplify magic. It can even be used for protection or to send a spell. The device itself is not magical. Shambles are like spectacles, they help you see, but don't see for you. A conversant witch can assemble a shamble in a matter of seconds using stuff like strings, twigs, leaves, feathers, beads, coloured paper, an egg or even a beetle. The whole thing looks like a "cat's cradle", or some sort of nest made of rubbish. The ingredients are not really important, although the centre should contain a live ingredient (e.g. an egg or a beetle. On one desperate occasion, a Nac Mac Feegle has been employed as the living part of a shamble).
The magic lies in its assembly and use, which is to catch the moment. "The way you tie the knots," said Miss Level, who was a Research Witch, "the way the string runs - the freshness of the egg, perhaps, and the moisture in the air - the tension of the twigs and the kind of things that you just happen to have in your pocket at that moment - even the way the wind is blowing. All these things make a kind of... of picture of the here-and-now when you move them right."
The most effective shamble is the shamble assembled when the witch really needs one, as Tiffany Aching discovered, who could never make one before. If Granny Weatherwax is to be believed, she never made one at all. Couldn't get the hang of them. They got in the way.
Some witches assemble extra shambles or even buy some just to hang around for decoration; these shambles are generally not very useful, but some of these still seem to react to strong magic. When the ambient magic is really too strong, the shamble explodes.
According to the Wizards a shamble is just a crude way to ensure thaumic cusping in phase-space, but what do they know? In fact, though it sounds like a rationalisation for the random assemblage at the bottom of many handbags, plus a lot of copper coinage, a shamble seems to be a practical tool compared to the very rare and expensive thaumometer favoured by the Faculty.
Hex, the organic/inorganic/magical super-computer in Unseen University's High Energy Magic Building tended by Ponder Stibbons and a group of like-minded, spotty, if-only-we-had-anoraks undergraduates can be considered something of a 'Super-Shamble'. Its initial components were a mouse-wheel and an ant-colony (the sum in this case is far greater than the parts).