Answer:
In last two lines (i.e. line 13 and 14) there is a twist that tells the reader that the speaker accepts and loves his mistress despite all her supposed flaws.
Step-by-step explanation:
Twist or Volta (Italian word for turn) is a rhetorical device that represents dramatic shift in in thought and/or emotion. Petrarchan or Italian sonnets have volta between the octave and the sestet while Shakespearean or English sonnets usually have it before the final couplet
The speaker also declares his love for his mistress in line 9, but here he just says that he loves to hear her speak. And just in next line (10) he says that music is pleasing than her voice.
By this method (mentioning his mistress's many flaws ans still loving her), the speaker makes his love even more valuable and rare. The poet also criticizes other poets for making false and exaggerated claims about beauty of their mistresses.