The “ison” of the Byzantine music is a held sound which, having got loose from a melody, is then of use as base. The western Middle Ages often took back this principle which represents one of the most primitive step of the polyphony, and variously developed it.
The generative idea and the diverse developments of this work feed on this process.
So for example, in the beginning, the series of held chords propelled by fast patterns and pulled over an unchanging bass.
Or still, in the central part, these notes which freeze alternately in a deployment of crystalline tones.