Yoreh Deah Siman 20 Seif 3

ג צָרִיךְ הַשּׁוֹחֵט שֶׁיִּשְׁחֹט בְּאֶמְצַע הַצַּוָּאר; וְאִם שָׁחַט מִן הַצְּדָדִין, שְׁחִיטָתוֹ כְּשֵׁרָה, וְהוּא שֶׁהֶחֱזִיר הַסִימָנִים וְיוֹדֵע שֶׁחֲתָכָם קֹדֶם שֶׁחָתַךְ הַמַּפְרֶקֶת, כִּי הַסִימָנִים רַכִּים וְנִדָּחִים מִן הַסַכִּין; וְהוּא הַדִּין לְשׁוֹחֵט מִן הָעֹרֶף.

 

Seif 3

A slaughterer needs to slaughters it in the middle of the neck; and if you slaughter on sides of the neck, the slaughter will be acceptable, (only) if he turned the signs back and he knows that he cut them before cutting the neck bone, because the signs are soft and they get pushed away from the knife, It is the same halacha if you slaughter it from the back of the neck.