Yeh I came across a sentence recently that basically stated that you can not turn left here, turn right.

So the turn left was поворачивать as it is the imperfect aspect, and you are never able, at any time, to turn left. Then it used the imperative form of повернуть (which is the perfective) to state turn right, as this is an action that will take place, and be completed in the near future (as perfective only has future/past rather than present).

I think the way I was taught to use perfective/imperfective is like if you read a book in the past but didn't finish it (you were reading) then you use the imperfective past, if you completed the book, (you read it) then you use the perfective past. This can be translated to the future tense where you can say I will be reading, I will read and complete.