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 are chosen by a Random Number Generator. There is a Random Number Generator in the machine's software.
The rate of generating random numbers by the RNG is extremely high. 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. A fraction of a second earlier or later, and the result would be different.
The RNG was noticed to generate not exactly random numbers. Most RNGs will repeat their number sequence. This behavior is due to poor programming. It is relatively easy to build pseudo RNGs with very long periods. No computer can complete a single period in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. Equations for specific gambling games like Keno were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the player cannot see the secquence.

Payout Percentage
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. This payout percentage is called theoretical. Jurisdictions serve to control the minimum theoretical payout percentage. Law or regulation typically establish it. A certain winning pattern exists nearly in every casino. The amounts they pay and the frequencies of these pay-outs are reflected in it. 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 players get the rest of the money.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. Based on current technology, this is a time-consuming process and as such is done infrequently. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. Gaming Control Board officials should be called for changing it. Other jurisdictions constantly ensure that slot machines 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. The change can be done only after the selected machine has been idle for at least four minutes. 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
A group of machines offers a particularly large prize, or jackpot if they are linked together in a special way. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
There can also be multiple casinos consisting of multiple machines. 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 it is possible to make the slot machine frequently display combinations that are close to winning combinations.
The term near-miss programming is used to denote this practice.
This term is also used for a related phenomenon. 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. There are roughly equally randomly distributed combinations above and below the pay line. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines 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 the subject of the Nevada Gaming Commission investigation. If the near-miss above or below the pay line was not specially programmed it was stated to be legal. That means that the possibility of near-miss occurrence should be equal to the possibility of any other combination. The machine can't 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. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.

You must know exactly the game you are playing.
Only the correct amount of money should be inserted into a machine. Always cash out your credits before leaving a machine.
While playing just have a good time. If you feel lucky increase your bets and vice-versa.