What do we call the shift in accents and metrical groupings from triple to duple?

The transition from a triple to a duple meter is known as hemiola. This term specifically refers to a rhythmic pattern that involves a shift where two beats are played in the time typically occupied by three, creating a sense of rhythmic tension and excitement. In a practical sense, this can be observed in music where a passage originally felt in triple time (like 3/4) suddenly feels like it's being grouped into two beats (like 2/4), thereby altering the listener's perceptual experience of the music's rhythm.

Hemiola often adds an intriguing complexity to music and can be found in various genres, particularly in classical and folk music, where it lends itself to dynamic rhythmic interplay. It effectively allows composers and performers to play with perceived rhythm, offering a heightened sense of movement and energy within a piece.