Random Number Generator
Nowadays slot machines are computerized. The odds are programmed. In modern slot machines, the reels and lever exist only for historical and entertainment reasons. The positions of the reels depend on a Random Number Generator. The machine's software includes a Random Number Generator.
Random numbers are constantly generated by the RNG at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. This means that the result varies depending on exactly when the game is played. The result changes every fraction of a second.
The numbers generated by the RNG seem to be not exactly random. Indeed, most RNGs will eventually repeat their number sequence. Poor programming is its reason. Pseudo RNGs with very long periods canbe built relatively easily. There is no computer that is able to complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. Equations for specific gambling games like Keno were discovered by him. The fact that the next set of selected numbers would be based on the previous games played was revealed. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the numbers can't be exactly found in a sequence.

Payout Percentage
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. The minimum theoretical payout percentage varies among jurisdictions. Its establishment refers to the sphere of law or regulation. A certain winning pattern exists nearly in every casino. This is the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The rest of the money goes to the players.
A slot machine's software is written and the theoretical payout percentage is set simultaneously. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. Being a time-consuming process it is done not very often. In certain jurisdictions the EPROM has a tamper-evident seal. Gaming Control Board officials should be called for changing it. The audit of slot machines is made by some jurisdictions make sure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. 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 Linking
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. 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. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice of showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
This term is also used for a related phenomenon. It is winning percentages that 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. The occurance of this phenomenon is possible when abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines, virtual reels are used 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. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. The near-miss and any other combination should have the same possibility of occurrence. The machine can't show winning combinations more frequently than other combinations above or below the pay line.
Near-miss programming, where a near miss is inaccurately displayed is not allowed in Australia either. Stop motion cameras are used by regulators for manufacturer's practices audit.

Slot Machines Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to 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. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.

I tis important to now what you are playing.
Only the correct amount of money should be inserted into a machine. Always cash out your credits before leaving a machine.
Only if you have fun your gambling won't be stressful. If you feel lucky increase your bets and vice-versa.