Answer:
The final couplet reverses the ideas of the three quatrains.
Step-by-step explanation:
Just look at some examples:
"Yet eyes this cunning want to grace their art,
They draw but what they see, know not the heart." (Sonnet 24)
"Yet seem’d it winter still, and, you away,
As with your shadow I with these did play." (Sonnet 98)
"And yet, by heaven, I think my love as rare
As any she belied with false compare." (Sonnet 130)
As is seen in these couplets, they reverse the ideas of their sonnets. The conjunction yet is used to introduce a phrase or clause contrasting with what has already been said.