>>10159319>>10159342 is sort of right. Here's how it actually works: In each slot machine there is a USB-like stick device attached to the hardware. This device contains the game program, often-times written in linux though sometimes not. Each slot machine functions in the exact same way. The only difference is minor bonus features or graphic art style.
What the program does is cycle through thousands, if not hundreds of thousands of number combinations per second in a continous loop regardless of whether the machine is in play or not. Each possible reel combination corresponds to a particular reel combination. My guess is there is an exponential relationship between the probability of higher payouts. The higher the payout, the expoentially less likely you will land on it.
In any case, the moment the player slams the spin button, the program selects the number combination currently being cycled through in the code. The machine has already determined what the reels will look like even before the graphics begin moving to show you the results.
As the other guy said, it is probabe to spin 10 spins and win 10 grand jackpots. However, the likelihood of that happening in practical terms is zero. Slot machines use these proabilities to pyschologically addict players into thinking the next spin might be the big one, so they keep going.
It's no different than World of Warcraft making Gorilla Meat have a 20% chance to drop on a mob you need for a quest. The next gorilla kill might have the meat you need, therefore keep playing and paying blizzard money.
