Sonnerie au passage is a striking complication that sounds the hours automatically, without the wearer needing to press any button. It differs from the minute repeater precisely for this reason: it is not on demand, but spontaneous. The watch decides when to strike — every full hour — and it does so on its own.
The mechanical principle is more subtle than it appears. During the entire hour, the minute wheel accumulates energy in a spring or lever mechanism; when the sixtieth minute strikes, that energy is suddenly released onto a hammer, which strikes the gong or bell. In the MeisterSinger Bell Hora the process is clearly visible: the hammer loads progressively and the final strike is sharp, not gradual. In the Chopard L.U.C. Strike One the hammer strikes the inner edge of the sapphire crystal, using the case itself as a sounding board — an ingenious solution but one that makes the sound highly dependent on wrist positioning. Christopher Ward's Bel Canto integrated the complication on an FS01 module mounted on a modified Sellita SW200 base: over sixty additional components for a solution that brought this type of striking into unconventional price ranges.
Historically, sonnerie au passage was the practical answer to the absence of reliable artificial light: eighteenth and nineteenth-century pocket watches used it to inform the user without looking at the dial. It is not a simplified form of the minute repeater — they are conceptually different complications. The minute repeater strikes on demand and announces hours, quarters, and minutes; sonnerie au passage strikes spontaneously and generally only the hours, sometimes the quarters.
All calibers with this complication feature an on/off function, which may be found on a separate pusher, on the crown, or in an opening on the dial. This is a necessary precaution: automatic striking in a meeting or at night is not always welcome. Deactivation does not block the energy accumulation mechanism, but prevents the hammer from being released — a non-trivial constructive detail.
I am not aware of documented cases of sonnerie au passage with quarter striking on a regular production wristwatch: if they exist, they are prototypes or one-off pieces. Those who discuss this with me usually confuse this complication with the grande sonnerie, which is something entirely different.