A tourbillon mounted at the centre of the movement rather than offset in a sub-dial.
A central tourbillon is a tourbillon mounted at the centre of the movement, rather than offset to one side, most commonly at 6 o'clock, as on a standard tourbillon. Rather than sitting in its own sub-dial, the cage takes the place normally occupied by the hour and minute hands, or dominates the dial from the middle of the watch.
Because the tourbillon cage has to clear the centre of the movement, where the hands are usually driven from, a central tourbillon typically demands a larger cage and a reworked gear train to carry the time display around or past it. That added engineering, combined with the visual prominence of a mechanism sitting front and centre rather than tucked into a sub-dial, is usually the point: central tourbillons tend to be built as a statement piece rather than a subtle addition to the movement.
Kerbedanz's Maximus is a well-known example, with a central tourbillon cage 27 mm across, officially the largest tourbillon fitted to a wristwatch.
Placement. A standard tourbillon sits in its own sub-dial, usually at 6 o'clock. A central tourbillon is mounted in the middle of the movement, in the space usually occupied by the hands, and is typically larger and more visually dominant as a result.
Mostly for visual impact. Moving the cage to the centre of the dial makes it the focal point of the watch rather than a detail in a sub-dial, and the extra engineering it demands, reworking the gear train to carry the time display around the cage, is often itself a display of technical ability.
Kerbedanz's Maximus houses a central tourbillon cage 27 mm across, the largest tourbillon ever fitted to a wristwatch.