Verified answer

They used to be mechanically operated but these days, they are all computer controlled.

Taken from HowStuffWorks:

Each possible combination is assigned a number, or numbers. When the random-number generator receives a signal -- anything from a coin being dropped in to the handle being pulled -- it sets a number, and the reels stop on the corresponding combination.

Between signals, the random-number generator operates continuously, running through dozens of numbers per second. This has two practical effects for slot players. First, if you leave a machine, then see someone else hit a jackpot shortly thereafter, don't fret. To hit the same jackpot, you would have needed the same split-second timing as the winner. The odds are overwhelming that if you had stayed at the machine, you would not have hit the same combination.

Second, because the combinations are random, or as close to random as is possible to set the program, the odds of hitting any particular combination are the same on every pull. If a machine is programmed to pay out its top jackpot, on the average, once every 10,000 pulls, your chances of hitting it are one in 10,000 on any given pull. If you've been standing there for days and have played 10,000 times, the odds on the next pull will still be one in 10,000. Those odds are long-term averages. In the short term, the machine could go 100,000 pulls without letting loose of the big one, or it could pay it out twice in a row.

The random number generator hits before the wheels even move, the slow wheels are just for show, as the outcome has already been chosen. Typically the others of one wheel are different from the other, which is why often times you will hit 2 out of 3.

You need to be the programmer of the game to know the real deal on how slot machines work inside. I think you can have a lot of answers about this if you find time to Google it.

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