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As well as two-motion selectors, there's other equipment in an automatic Strowger exchange.
Ringing Machines are generators driven by an electric motor. Mechanical cams on the motor shaft open and close electrical contacts at various speeds, producing signals at various frequencies applied to lines by the two-motion selectors to indicate call status — dial tone, ringing tone, engaged tone, congestion tone (more properly "Equipment Engaged Tone," used when no path through the exchange can be found), and Number Unobtainable. The video shows a ringing machine generating both ringing tone (sent to the caller) and ring current (sent to the called party). Exchanges run on Direct Current; ringing the bell needs Alternating Current, also generated by the Ringing Machine.
Another essential piece of kit is the interface between the line and the exchange: a Line Circuit, consisting of three relays and a uniselector. The uniselector works a bit like a two-motion selector, but instead of stepping up then rotating, it just rotates — you might call it a "one-motion selector." Its job, controlled by the three line relays, is to connect the caller to a free two-motion selector when they lift the handset. Two-motion selectors were big, expensive pieces of kit, so having one for every line (mostly unused at any given moment) would be wasteful. Instead there was one for each call anticipated during the busiest period of the day — "subscriber concentration," sizing switching capacity to anticipated traffic, not every customer, is an idea still used in modern digital exchanges.