Castanets may look like two pieces of wood being knocked together, but their shallow bowl or shell shape is essential. Two hardwood shells face each other, loosely joined by a cord. Their outer rims make most of the contact, while the hollow centre leaves space for the wood to vibrate freely. The result is short, bright and dry, rather than the dull thud of two solid wooden blocks.
Traditional handheld castanets are usually played as a pair in each hand. The higher pair often sits in the right hand, where the fingers can articulate a rapid roll, while the lower pair in the left hand marks a steadier pulse. What matters is not simply the loudness of the click, but the clarity between particles: a good roll sounds connected without becoming a blur, and the accented stroke gives the dance its direction.
In an orchestra, castanets may be mounted on a handle or base so a percussionist can pick them up quickly and keep a reliable rhythm. The mechanism gives up some of the fine control of the fingers but gains consistency and speed of access. Composers often use the sound to evoke Spanish dance, yet the instrument is more than an ethnic label. Its musical value lies in how tiny changes of motion become audible rhythmic definition.
There is a useful lesson here for pianists as well. A fast group of notes should not be pressed into one mass. Like a castanet roll, each attack needs its own edge while the gestures still point in one direction. Flow comes from clear particles, not from blur.
https://sites.duke.edu/dumic/instruments/percussion/west-europe/castanets/
https://americanhistory.si.edu/collections/nmah_607015
https://data.fitzmuseum.cam.ac.uk/id/terminology/term-93337