The primes >3 are all of the form 6k+1 or 6k-1, so they are close to 6k. As k increases, the resulting marks on the polar coordinate system form nearly complete turns because 6 is close to 2 pi. Hence, spirals. See my other comment for a clearer explanation of why primes >3 all must necessarily be one more or one less than a factor of 6k where k is an integer. That is the crucial piece that the author did not make totally clear.