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)! He needed to give her reasons that she could fathom. The Pesik (stroke) after va'Yema'en hints that the following reasons were not why he refused.
This is because the Yetzer ha'Ra, who is an angel, will flummox one by presenting counter-arguments for consenting to his request. Whereas a blunt 'No!' will leave him flummoxed.
Why is the Trup (note) on va'Yema'en a 'Shalsheles'?
Peninim mi'Shulchan ha'Gra, from Pesikta Zutresa: It hints to refusal after refusal. Yosef screamed three times that he refuses to give in to the Yetzer ha'Ra.
Penei 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. Bechayei: He refused with absolute finality.
Alshich: He refused and considered that he had tied himself in a Shalsheles (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].
