It is referred to as 'jo' (prelude) or 'naminobu' (wave section), in which koto expresses waves coming and drawing, and kokyu expresses the sound of the wind through pines with chords.
「序」とか「波の部」と呼ばれ、箏で波が寄せ返す様、胡弓が和音で松風の音を描写している。