Why Can the Same Six Pulses Suddenly Sound Like Three Groups of Two?

A hemiola most often does not change tempo or add or remove duration. It regroups the same six underlying pulses. What first sounds like two groups of three — 1-2-3, 1-2-3 — can be accented as three groups of two — 1-2, 1-2, 1-2. The notation may remain in triple metre while the perceived metric weight briefly shifts towards duple metre.

The effect has force because the previous metre does not vanish. The listener still remembers the established triple framework, so the new duple grouping creates tension against it. Baroque dances often use this regrouping near a cadence. Rather than stopping the phrase, it compresses several bars into one longer push. In the first movement of Mozart's Sonata K.283, around bar 7, the left hand similarly lets six quavers form a weight that cuts across the written bar line.

At the piano, the answer is not simply to strike the first note of every new group more loudly. The six pulses must keep flowing evenly while harmony, bass and voice-leading reveal the changed grouping together. If the underlying pulse breaks, the hemiola sounds like an accidental change of speed. If every note retains the old triple emphasis, the effect disappears. What we should hear is the same span of time being reorganised, not music adopting a new tempo.

Sources:
https://openmusictheory.github.io/sentence
https://imslp.org/wiki/Piano_Sonata_No.5_in_G_major%2C_K.283%2F189h_(Mozart%2C_Wolfgang_Amadeus)