Slot machine is a very simple, powerful electronic machine. All functions of the slot machine are controlled by a motherboard plug-in, just like the computer you are sitting at. All functions of the slot machine, including the payout percentage, are controlled by this plug-in board. This board is installed at the factory, according to specifications ordered by the customer. Every aspect of design, manufacture, installation, and maintenance of slot and electronic machines is regulated and monitored by state gaming boards. The gaming boards only allow boards to be swapped out under their supervision. In other words, the machine on the floor is the same one that passed their inspection upon design, assembly, and installation. Any future change, by the casino, must also pass muster. Generally, Indian casinos are not regulated by these state boards.
The central processing unit (chip), and the board it is attached to, give orders to the machine. According to the program installed at the factory, it commands: when to spin the reels, when to stop the reels, whether to credit or debit the player from the credits already fed into the machine and how much and when to pay on a jackpot. If the player is using a slot club card, another line of communication runs to the slot club computer, detailing how much is being played, how much is lost, and how much is won. This slot club record has no affect on the outcome of the slot pull.
The slot machine works according to the set of instructions issued by the motherboard. This motherboard contains a random number generator (RNG). This program is installed at the factory and determines when jackpots occur according to the payout percentage on that particular machine that was installed at the factory. Casinos expect machines to return their theoretical payback only over a very large (hundreds of thousands) number of pulls. The function of the RNG is to "seed" the program with a "random" pull when you push the "play" button. Slots is a game of chance, after all, and the RNG assures that it remains so.