The RNG
Modern slot machines are computerized. They programme the odds. The reels and lever are present for historical and entertainment reasons only. The positions of the reels depend on a Random Number Generator. It is included into the machine's software.
The RNG generates random numbers at an extremely high rate. The most recent random number is used to determine the result when the Play button is pressed. This means that the result varies depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
It was noticed that the RNG does not actually generate random numbers. Most RNGs will repeat their number sequence. Poor programming determines such behavior. To build pseudo RNGs with very long periods is relatively easy. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. They allowed to predict that the next set of selected numbers would be based on the previous games played. However even if nobody uses the machine the RNG continues to pick numbers. So the player cannot see the secquence.

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 is known as the theoretical payout percentage. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation are used to establish it. A certain winning pattern is determined by every casino individually. The amounts they pay and the frequencies of these pay-outs are reflected in it. The winning patterns on slot machines are carefully selected to yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
When a slot machine's software is written its theoretical payout percentage is also set. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. Being a time-consuming process it is done not very often. There is sometimes a tamper-evident seal on the EPROM. It can only be changed in the presence of Gaming Control Board officials. Other jurisdictions constantly ensure that slot machines 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. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. After the change is made, 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.

Machines That Are Linked
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. 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.
Multiple machines can be sometimes linked across multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is under computer software's control. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice is called near-miss programming.
There is a related phenomenon that is also sometimes called near-miss. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. However, the combinations appearing above and below the pay line are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. This phenomenon only occurs where 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 issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. They ruled that this was legal, so 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. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. To audit manufacturer's practices regulators use stop motion cameras.

Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.

There are at least two types of slot players.
Persistent player.
The players of this type believe that very soon there will be a period when the machine begins to pay. The machine can begin paying in any second.
The player will play until the machine pays out. He must have found a machine that haven’t paid for a long period of time or he has inserted to many coins and has got nothing. They believe that at least any payout is due to happen. They can’t leave the machine because as soon as they do it will burst forth and pay out large sums. Maybe it can really work if you have already spent thousands of dollars and have got nothing back. The machine can’t but paying.
Many years ago in the case of the payout players touch the coins to check whether he coins were hot or cold. If the coins were hot they believed that the machine had not paid for a long period of time. The heat of the machines lights, etc would eventually heat the coins.
The chaser.
The player thinks that the machine he is going to play is not a machine that never pays. Players feel more comfortable and confident because they have been reinforced with a win and gain trust in the machine. A sense of comfort exists between the slot machine and the player.