There are grammatical mistakes that have to be corrected first, and then the sentence will make more sense. "Workouts divided into 20-minute chunks with 10-minute rest periods may be optimal for you, particularly if the quality of focus is quite high." You cannot use "divided" without the noun it describes, so I added "workouts" based on context. Finally, by itself, "Particularly if the quality of focus is quite high." is not a sentence because it contains the "if" conditional without explanation. I'm not 100% sure what you mean by "focus", though. Maybe "intensity" would be a better word.

Here also, you see the kind of confusion that is made evident when an author starts to police your thoughts and emotions. Textbook authors are paid by the word. You should apprise yourself of that. This is one reason their written instructions may seem so absurd and tedious. IF...is a "conditional" It means that what follows is a POSSIBILITY. There is a possibility that your focus, in study, is "high" or "intense". The author proposes that you should study for about 20 minutes if you are an intense student. Of course, if you are a student with a mind that does not focus with "intensity" upon your subject, you might need to study for a longer period. IF, as a conditional, is not "probative". You need to know this if you study the law. EXAMPLE: I can write to you that you are a "crazy" person "...IF you think that Astronauts really landed upon the Moon; because I think it is all fake!" It is a rhetorical device known as a Logical Fallacy. (An error in logic). It is an error because there is no indication that you have ever discussed Astronauts landing on the Moon. Just the same, a very manipulative person will write that way because it gives them a chance to suggest that someone else is "crazy". So the author of your textbook is writing a "conditional" which is only true if it is also a fact that your "...quality of focus is quite high". It may be you have a low quality of focus, in which case, nothing this author writes applies to you. (I hope you understand this. If not, ask more.)