In this video:

https://youtu.be/jGI6JgKiX8w?t=72

...from 1:13 to 1:20 there is a special sound connection between the clarinet and the accordion. I don't know how to describe it but sounds like a very nice transition (specially if you listening from the very beginning of the song).

**My Questions**

What is that 'in tune' sound effect called?
Is there any branch of music dedicated to the study of the dynamics of song through it? I mean, how do they know that at that particular time, that effect would sound nice(for me sounds like all the stuff they play before that moment, sound enhance that "in tune" effect).