In the early 90s, this Madrid "gato" or "cat" discovered that certain roulette wheels were not completely random, but were biased due to tiny imperfections or flaws in the roulette wheel's gears, small differences in the pocket sizes, or because they weren´t mounted perfectly horizontally: so some numbers came up more often than others. Garcia-Pelayo analyzed in fine detail the statistical distribution of the wheel across thousands of spins, and hundreds of sessions and then built up a profile of the wheel with "hot spots" based on this raw data.
In simple terms what Gonzalo Garcia-Pelayo did was (he and members of his family) record the outcomes of say 5000 spins. The expected occurrence of each number (in European roulette) would be 135 appearances. If one or more numbers appeared more than three standard deviations, that is more than 170 times, then he considered the wheel biased and bet these numbers every spin.
Now take a cold beer, sit comfortably and enjoy this wonderful documentary about the story of Gonzalo Garcia-Pelayo. Perfect for a hot and lonely summer night.