From what I understand, polyrhythms are basicly the same as 'beats', which is the effect produced when two frequencies that are close, but not the same are played together. The result is a beating sound, as the frequencies match up and drift apart.

A real life exaple is helicopter rotors:
As the blades rotate, the blade moving away from you is dopler-shifted to a slightly lower frequency, and the blade moving towards you is dopler-shifted to a slightly higher frequency (just like a police siren passing you - first it is high pitched, then low once it passes you). The combination of the slightly seperated frequencies produces the beating sound that we recognise as a helicopter.