The RNG
There are many modern computerized slot machines. The odds are programmed. The reels and lever are present for historical and entertainment reasons only. A Random Number Generator determines the positions of the reels. The machine's software contains it.
The RNG is constantly generating random numbers at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. The result varies depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
Some professional gamblers observe that the RNG does not actually generate random numbers. The sequence of numbers is repeated by most of the RNGs. Poor programming is its reason. To build pseudo RNGs with very long periods is relatively easy. There is no computer that is able to complete a single period in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, had access to the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. Thanks to them the fact that the next set of selected numbers would be based on the previous games became known. But the RNG picks numbers even when the machine is not being played. So you can't find an exact place of a number in the sequence.

Percentage Of The Payout
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. This payout percentage is called theoretical. Jurisdictions influence the minimum theoretical payout percentage. Law or regulation are used to establish it. Nearly each casino has its own winning pattern. It contains the information about the amounts they pay and the frequencies of these pay-outs. The winning patterns on slot machines are carefully selected to yield a certain fraction of the money played to the house. The rest of the money goes to the players.
When a slot machine's software is written its theoretical payout percentage is also set. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. It is done infrequently because this is a time-consuming process. There is sometimes a tamper-evident seal on the EPROM. To change it Gaming Control Board officials should be present. Other jurisdictions randomly audit slot machines to ensure that they contain only approved software.
Thanks to the newly developed technology the game, the odds, and the payouts would be able to be changed by the casino's slot manager remotely. Only after the selected machine has been idle for at least four minutes can the change be done. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Machines That Are Linked
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
Sometimes multiple machines form multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
Computer software controls the reel display of modern slot machines. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
This practice is called near-miss programming.
There is a related phenomenon which is also called near-miss. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. The combinations above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. The occurance of this phenomenon is possible when abbreviated physical reels are used to display a win pattern based upon the RNG. Video slot machines have virtual reels and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The Nevada Gaming Commission investigated the issue of a near-miss above or below the pay line. It was considered to be legal as long as the near-miss above or below the pay line was not specially programmed. In other words, any other combination must be just as likely to occur as the near-miss. The machine cannot be specially programmed to show winning combinations more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. Due to this, regulators use stop motion cameras to audit manufacturer's practices.

Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors were changed for bill acceptors. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.