hello

@Karthikeyan, I don't really get your question but let me try. I am using Kaplan to prep. And I've learnt that sentences contain key words and key phrases that signal connections between ideas. Some are used to corroborate/support/elaborate the idea e.g and, since, also thus etc. while some change the direction of an idea e.g but, despite, yet, however, unless. For what you posted, I think those key words and phrases are what is being referred to - SIGNAL WORDS. The shift sentence indicate the SIGNAL words are going to change direction of an idea (but, despite, yet, however etc)

For the "strong" ones they are easy to detect - that they are indicating a change in direction of idea (like the first two examples you posted). The "weak" ones are not easy to detect-that they change/contrast an idea (like in the last example )