How can two rhythms play at once without the music falling apart?

Two rhythms can play at once without the music falling apart because, as scholars of West African rhythm describe it, they are not really two separate things. A polyrhythm is the simultaneous use of two or more rhythms that are not readily heard as deriving from one another, and the simplest case is 3 against 2: three evenly spaced beats in the time normally occupied by two. Musicians call it a hemiola.

The scholar Victor Kofi Agawu puts the key point this way: there is no independence here, because 2 and 3 belong to a single Gestalt. The two beat schemes interact inside one metric hierarchy, one primary and one secondary. In sub-Saharan African rhythm the dancer's feet typically mark the primary beats, while the secondary beats are accented musically, so the body keeps the ground while the sound crosses it.

You can try it yourself with a memory aid: say "not difficult" over and over. The beats of both layers fall together on "not", the second and third beats of the three-beat layer land on "dif" and "cult", and the second beat of the two-beat layer lands on "fi".

Eugene Novotney observes that the 3:2 relationship and its permutations are the foundation of most typical polyrhythmic textures in West African musics.