Pull or push the slide and the repeater's own spring is wound; release it and a train governed by a fly regulator drives racks and snails that measure the current time and release the correct number of hammer blows.
The hours are read from a stepped hour snail, the quarters from a quarter snail, and in a minute repeater a further star-and-snail arrangement counts the minutes past the quarter. The racks fall onto these snails, and their travel determines how many teeth pass the hammers' lifting pins.
Governing the speed matters as much as counting. Without a fly or centrifugal governor, the strike would run too quickly to be intelligible; the even, measured cadence of a good repeater is a mark of correct adjustment, not decoration.
Repeaters are unusually vulnerable. Operating the slide slowly or interrupting the strike can leave racks partially engaged; pulling the slide while setting the hands can jam or break the all-or-nothing piece, a safety part specifically designed to prevent partial strikes.
This is specialist work. A repeater should not be dismantled as a learning exercise, and even cleaning requires a watchmaker experienced with striking work. Incorrect reassembly of the rack and snail relationship produces a watch that strikes the wrong time.
For collectors, the research questions are whether the mechanism is complete and original, whether the gongs and hammers are correct and unbroken, and whether the movement and case belong together — complicated movements were frequently re-cased.