The in a sonnet is the shift in the central ideas.

I believe the answer is the Volta.
The volta is characterized by sudden dramatic change in emotion or train of thought. It's a very crucial part for the sonnet because it move the story forward and bring it closer to the climax. In English sonnet, Volta usually occurs before the final couplet.

It could be volta. The main its meaning is the turn in thought in a sonnet that is often indicated by such initial words as But, Yet, or And yet.