Skeletonization is not a decorative effect added afterward: it's a constructive choice that shapes the movement's design from the start. Material is removed from the mainplate and bridges until only structurally necessary sections remain, then the surface of what's left is finished — anglage, polishing, satin brushing — so every edge becomes legible to the naked eye through the crystal.
There are different degrees. The open heart is the minimal case: a shaped opening in the dial, often centered on the balance wheel, that reveals the beat without affecting the rest of the structure. A skeletonized dial removes the traditional disk and uses the movement's bridges as graphic elements — the Delfin Mecano case with its matte green hourglass bridges is an extreme example. Complete skeletonization of mainplate and bridges, as in the Vacheron Overseas Perpetual Calendar Ultra-Thin Skeleton or the Code41 Mecascape, brings visibility to both sides of the case: front crystal and sapphire caseback show the same mechanics from opposite angles.
The display back — transparent caseback — is technically different: it requires no material removal from the movement, just replacing the solid caseback with glass. It's useful for showing an automatic caliber with finished surfaces, but it's not skeletonization. The distinction matters because the two approaches have completely different costs and structural implications.
From the bench I've seen skeletonizations of very different quality. When the work is done well, every edge is hand-beveled and the remaining bridges have constant cross-sections. When done poorly, the cut edges are rough and the geometry is random. On an open movement you can't hide anything: skeletonization is also a test of constructive honesty.
A flying tourbillon mounted on openwork movement adds another element: without a top bridge, the cage appears free in space, which works visually only if the surrounding skeletonization is airy enough not to compete with the main element.
A detail often overlooked: on fully skeletonized movements, lubrication is more critical because the bearings are exposed to dust and humidity variations more than they are in a closed caliber. Anyone wearing a skeleton watch outside controlled environments should keep this in mind.