The classic slot machine design works on an elaborate configuration of gear and levers. The central element is a metal shaft, which supports the reels. This shaft is connected to a handle mechanism that gets things moving. A braking system brings the spinning reels to a stop, and sensors communicate the position of the reels to the payout system. A coin detector initially registers that a coin has been inserted and unlocks a brake so the handle can move.
There are a number of ways to arrange these elements, and manufacturers have tried dozens of approaches over the years.