Skeletonizing is a method of refining watches
In order to give the watch a more filigree appearance, bridges, plates, dials and possibly also rotors are cut out, leaving as little material as possible, so through watch glass and transparent case bottoms an elaborate ornament is visible.
This is particularly complicated with chronographs, because there are considerably more parts to be processed than with simple watches.
Skeletonized movements are often provided with fine engraving and chiselling.
There is manual and automatic (computer controlled) skeletonization. Best known for the former are the skeleton watches by Kurt Schaffo; these are even unique to each. As a young, emerging talent the gifted East German watchmaker Stefan Kudoke has emerged.
Note that the term, "skeleton", referred to any watch with visible components. As late as 1960, definitions specifically mention watches with transparent dials or display casebacks rather than just those with a cut-out movement.
Audemars Piguet and Girard-Perregaux were pioneers in skeletonization of movements, delivering what we would today recognize as skeleton pendant and pocket watches in the 1950s. But it was not until the 1990s that skeleton wristwatches were first delivered.