Since the Torah goes on to give a detailed account of Yosef's refusal, why does it begin with the word "va'Yema'en"?
Sefas Emes: When one is tempted by the Yetzer ha'Ra to sin, one must first of all say 'No!', before presenting reasons for one's refusal. 1
Ha'amek Davar: He himself refused to have Bi'as Zenus (even with a single woman)! But he needed to give her reasons that she could fathom. The Pesik (stroke) after "va'Yema'en" hints that the reasons that he subsequently gave were not the real reasons for his refusal.
This is because the Yetzer ha'Ra, who is an angel, will present counter-arguments for consenting to his request. Whereas a blunt 'No!' will leave him silenced.
What is the significance of the 'Shalsheles' (the Trup note) on the word "va'Yema'en"?
Peninim mi'Shulchan ha'Gra, from Pesikta Zutresa: It hints at refusal after refusal. Yosef screamed three times that he refuses to give in to the Yetzer ha'Ra.
Pnei David: It corresponds to the three Aveiros that he was refusing to transgress - Arayos, betrayal of his master, and Chilul Hashem (Chatasi le'Elokim).
R. Bachye: He refused with absolute finality.
Alshich: He refused and considered that he had tied himself in a Shalsheles (a chain - i.e. as if he had no choice).
What is the difference between 'Lo Avah' and 'Mi'en'?
Malbim: 'Lo Avah' is [refusal] in the heart. Mi'en is verbal. Even though his heart desired, he conquered his Yetzer ha'Ra [and said no].
