From http://www.music.vt.edu/musicdictionary/texth/Hemiola.html

hemiola - In early music, this term meant the ratio of 3:2, employed musically in two senses: the ratio of the perfect fifth, whose musical value is 3:2, and the rhythmic relation of three notes in the time of two, i.e., the triplet. In the Baroque era hemiola was used in dance music in the sense that it denoted the articulation of two measures of triple meter as if they were three measures of duple meter. In later music, especially Viennese waltzes the use of hemiola was common, in the sense of playing duplets in one part of the music, over which another part of the music is playing triplets.

3:2 Hemiola Exercises

Three Against Two Slow Shuffle

In exercise 2, perform the flam as a flat-flam (both hands same stroke, same time).