The odds behind the slot machines are determined by a replaceable microchip and the player usually has no idea how that microchip affects the outcome of the game. The only exceptions are when the casinos advertise a particular payback percentage, like 98.7%, and in Atlantic City payback percentage is mandated by the gaming authorities to be no less than 83%.
The way modern slot machines work is that they are constantly determining random numbers. According to Slot Machine Mania these random numbers vary from 1 to 4,294,967,296 (232). Most sources say that an optical sensor registers the moment you put a coin in and the random number chosen at that moment is used to determine the outcome. Other sources say that the random number is chosen the moment the handle or button is pressed to spin the reels. Once a random number is chosen the program microchip then uses that number to assign a particular outcome with a lookup table. It is how that lookup table is designed that determines how much the machine will pay out in the long run. After passing the random number through the lookup table and a particular outcome is assigned the reels will stop on exactly that outcome after the player pulls the handle.
Most books on slot machines incorrectly state or imply that there is a one to one relationship between possible combinations of the reels and possible random numbers. A ‘Triple Diamond’ slot machine I studied at the Atlantic City Tropicana had 3 reels with 22 stops on each reel, for total of 223=10648 possible combinations. Most books would falsely state that this machine would choose a random number from among 10648 and assign a unique set of stops to each one. If the reels had an equal chance to stop at every position that machine would pay 469% of money bet.