Somebody knows... Maybe you?
There was a clever pattern to it. Each display had two sets of symbols - a real set and a teaser set. The teaser set had more jackpot symbols on it to give the illusion of liberality, but it would never stop on them, only on the real set. The change from one symbol to the next as the game ran was in four stages - first the real, then both the real and the teaser at the same time, then just the teaser, then briefly dark. Persistence of vision left the player mainly remembering the teaser symbols, while the "tumble" effect came from seeing the (123-)(123-)(123-) sequence constantly repeated about eight times a second, too fast to make out but not too fast for the brain to notice the pattern.