From a Wired article (warning: adblocker-blocker, in-browser popups) via LWN:
[T]he operatives use their phones to record about two dozen spins on a [slot machine] they aim to cheat. They upload that footage to a technical staff in St. Petersburg, who analyze the video and calculate the machine’s pattern based on what they know about the model’s pseudorandom number generator. Finally, the St. Petersburg team transmits a list of timing markers to a custom app on the operative’s phone; those markers cause the handset to vibrate roughly 0.25 seconds before the operative should press the spin button.
The timed spins are not always successful, but they result in far more payouts than a machine normally awards: Individual scammers typically win more than $10,000 per day.
From Scarne's Complete Guide to Gambling, published 1974, on the "Rhythm Boys" scam from 1949:
During 1949 a couple of thousand rhythm players, most of whom were women, were beating the slots all over Nevada and various other sections of the country. Hundreds were barred from the slot rooms, and slander suits (which were later dropped) were filed by some of the barred players. My findings show that national slot machine revenue took a real nose dive, dropping, from the 1948 figure of $700 million to a rockbottom low of $200 million in 1949. The rhythm players beat the slots during 1949 for half a billion dollars.
How did the original mysterious stranger happen to come up with his bright idea? And who was he? I did some further detective work and discovered that he was an Idaho farmer who, during his spare time, had been helping a slot-mechanic friend repair out-of-order machines. He discovered that the three wheels in certain makes of machine made exactly the same number of revolutions when the handle was pulled. He studied the clock fan which controls the length of time the reels spin and found that on some machines the clock went dead from seven to eight seconds after the reels stopped spinning. He also memorized the position of each symbol on each reel. In actual play, since he knew the relative positions on the reel of all the symbols, he could deduce from the nine visible symbols he could see through the window just where all the others were. Then, by timing himself to pull the machine's level at precisely the right instant and before the clock gear went dead, he found that he could manipulate the desired symbols onto the pay line. Most of the rhythm players who learned the system later could, as a rule, control only the cherry on the first reel, but even that was good enough to empty the payoff coin tube; it just took a little longer.
Everything old is new again. Including exploiting insufficiently-random random-number-generators to make money in Vegas.