Nothing about a spinning reel is physical. The outcome is decided the instant you press the button, and the animation that follows is a way of showing you a result that already exists.
What an RNG is
A random number generator is an algorithm producing a sequence of numbers with no discernible pattern. Slot machines use a pseudo-random generator: deterministic given its starting state, but with a period so vast and a distribution so even that no practical analysis can distinguish its output from randomness.
It runs continuously, thousands of times a second, whether anyone is playing or not. Pressing spin takes the value available at that microsecond and maps it onto positions on each reel strip.
Why that matters
- The result is fixed before the reels stop. Stopping them early changes nothing.
- Spins are independent. The generator has no memory of what it produced. A game cannot be due.
- Timing gains you nothing. The generator cycles far faster than any human reaction, so the millisecond you press is effectively arbitrary.
Weighted reels
Here is the part that surprises people: symbols are not equally likely. Each position on a virtual reel strip appears a different number of times, so the top-paying symbol might occupy two stops out of two hundred while a low-value one occupies thirty. What you see on screen is five evenly-spaced symbols; what sits behind it is a weighted distribution tuned to produce a target return to player.
This is not deception. It is published in the game's mathematics and verified during certification. It does mean that counting symbols on the visible reels tells you nothing about their probability.
How one is tested
In a licensed market, an independent laboratory examines the generator's source, confirms the seeding is unpredictable, and runs statistical batteries over hundreds of millions of outputs looking for bias, periodicity or correlation. It then simulates the full game to confirm the stated RTP. The result is a certificate tied to a specific build.
That last detail matters: certification covers a version. A game changed after testing is a different game until it is tested again.