"Use. For each digit a push button from 1 to 9 is selected which rotates a Pascal-type wheel with the corresponding number of increments. Numbers are subtracted by adding the complement (shown in smaller numbers). The carrying of tens is accomplished by power generated by the action of the keys and stored in a helical spring, which is automatically released at the proper instant to perform the carry.
"Through effective marketing and training of skilled operators versed in complement arithmetic at Comptometer Schools, these machines became the workhorse of the accounting profession in the first part of the [20th] century. They never successfully advanced into the electro-mechanical era, but remained purely mechanical, two-function adding and subtracting machines" (Gordon Bell's website, accessed 10-12-2011).