Accelerating the harmonic rhythm
Accelerating the harmonic rhythm (of a passage or what have you) is what it sounds like: increasing the rate at which the harmonies (chords) change. Inversely, it’s decreasing how the duration of each chord.
For example, moving from 1 chord per bar to 2 chords per bar is an acceleration in the harmonic rhythm.
In general, this technique can be used to increase intensity.
In classical music, it’s used in the continuation phrase of a sentence to move the theme along.