Hello Elina, welcome to Using English!
It's difficult to know exactly what the author meant, without more context; but he seems to want to distinguish between a brusque "no", which is usually a simple negative, and a "no" where the vowel "o" is deliberately lengthened, which is usually a doubtful negative.
For instance, if you ask me whether I like tofu, I will say "no" in a brusque, definite, no-nonsense fashion, because I certainly don't like tofu.
If however you say to me "MrQ wasn't very amusing this evening, was he?", I might reply "No-o-o-o", in a "meditative tone", because it's true that MrQ wasn't very amusing; but I also know that there were extenuating circumstances, and so don't entirely blame him (his cat was run over by a bus yesterday).
This seems to mean "how do you pronounce 'please' in a way that emphasises its meaning?"
For instance, children often lengthen the "-ee-" sound, in an ingratiating way, e.g.
"Can I have another £50, please, Dad? "
"No, you can't. Now go away."
Let me know if it's still unclear!
Have a pleasant Wednesday,
- For Teachers