As technology advanced, manufacturers turned to optic verification sensors to prevent scams. These mechanisms use a beam of light to register payment as it's dropped in. Ironically, this technology was used against itself to perform a cheat very similar to the aforementioned yo-yo trick.
Intrepid ne'er–do–wells found that if a coin was slightly shaved around its edge, then a slot machine's optic sensor would register it as a normal coin. However, once it got to the machine's comparitor mechanism—the piece of equipment that measures size and weight—it would be kicked out because of the minute size discrepancy.
In many machines, the optic sensor worked independently from the physical comparitor mechanism. The former would be the sole judge of a coin's authenticity while the latter merely doled out change. Shaved coins were good for a play but would be returned in the change tray as bogus money—it's essentially the yo-yo trick sans string.