The vowel is close, the vowel is the same but not belongs to Duizhuan, rhyme tail is the same and the vowel is close, they are Heyun.

凡元音相近，或元音相同而不属于对转，或韵尾相同且元音相近，叫做合韵。

youdao

The gesture for this, the vowel most far forward in the mouth, is made by bringing the arms close together and parallel to one another, as if in making a tube.

U这个音是原音里口腔发音最靠前的一个。基本姿势是把双臂平行靠拢（上举或向下），就像做一个管道。

youdao