While most modern slot machines, look and feel like the old mechanical models, they actually operate quite differently than the original models. The new models are controlled by a central computer system located inside each machine rather than the motion of reels used by mechanical models. Using step motors, the computer turns and stops each reel at a predetermined point. Rather than the fluctuating electrical current that drives a standard electric motor, step motors are driven by short digital pulses of electricity that are controlled by the computer. With each pulse, the motor is moved incrementally with great precision. The games are not, however, preprogrammed to pay out at a certain time, even though the computer tells the reel where stop. Instead, a random number generator in the computer makes sure that each pull has an equal opportunity at hitting the jackpot.

Whenever the slot machine is running, the random number generator (RNG) is sending out whole numbers (usually between one and several billion) hundreds of times each second. The moment a player pulls the arm back or presses the button, the computer records the next few numbers from the random number generator. It then feeds these numbers through a simple program that will determine where the reels should stop spinning.

Since the RNG has already selected the stops, by the time the reels are spinning the game is essentially over. Thus, with modern slot machines, there is really no need for the reels to spin. They are there more for cosmetic purposes or as a courtesy to the player. In fact, computers have made the reels obsolete -- a player could just put his money in and the machine could simply tell him whether he won or lost. 

Some symbols are more likely to hit than others because the reels are weighted differently. For example, the slot may pick a random number from 1 to 27, which might correspond to the symbols in the following hypothetical example:

While this is not an actual chart of a slot, it does demonstrate the operating concept of the slot machines. If there are 22 stops on the wheel, which would include the 11 symbols and 11 blanks, you don’t have a 1 in 22 chance of hitting the jackpot -- you have a 1 in 64 chance of hitting it. This is very deceiving and, were casinos honest, they would hang a disclaimer on each machine pointing out the real odds of hitting a jackpot symbol.

Furthermore, each reel is weighted differently. Consequently, a player is much more likely to get jackpot symbols on the first and second reels, which builds the suspense level, but unlikely to get it on the third reel.

To understand how the process works with the standard three-reel machine, let’s assume that you are at a slot machine and have pulled the handle or pressed the button to get the machine started. At this point, the computer will record the next three numbers from the random number generator. The first number will be used to determine the position of the first reel, the second number to determine the position of the second reel and the third number to determine the position of the third reel. For this example, the first number will be set at 123,456,789.

To determine the position of the first reel, the computer must divide the first random number by a set value, usually 32, 64,128, 256 or 512. For this particular example, the computer will divide by 64. After dividing the random number by the set value, the computer then records the remainder of the quotient. In the example used above, the computer would find that 123,456,789, can be divided by 64 a total of 1,929,012 times with a remainder of 21.