Watch how mechanical calculators propagate carries through the register when a digit overflows from 9 to 0
On Odhner-type calculators like the Walther, when a counter digit goes from 9 to 0, a small wheel with a single tooth (the "hatchet") pivots outward into the path of the next pinwheel to the left.
This tooth adds one extra count to the next digit, propagating the carry. If that digit was also at 9, it too overflows, causing a cascade of carries.
The carry pins are cleverly staggered around the pinwheels so the rightmost digit is always processed first, ensuring any carry it generates can affect the next digit.