Yoreh Deah Siman 24 Seif 9

ט צָרִיךְ לִזָּהֵר כְּשֶׁאָדָם שׁוֹחֵט, וְחָס עַל הָעוֹר שֶׁלֹּא יַעֲשֶׂה בּוֹ קֶרַע גָּדוֹל, וְשׁוֹחֵט בְּרֹאשׁ הַסַכִּין וּמִתְכַּסֶה מֵהָעוֹר; אָמְנָם אִם שׁוֹחֵט בְּאֶמְצַע הַסַכִּין אֵין לָחוּשׁ אִם רֹאשׁוֹ מִתְכַּסֶה בָּעוֹר, כֵּיוָן שֶׁהַסַכִּין בַּמָּקוֹם שֶׁשּׁוֹחֵט בּוֹ כְּנֶגֶד הַסִימָנִים אֵינוֹ מְכֻסֶה; וְיֵשׁ מִי שֶׁמַּחְמִיר גַּם בָּזֶה, וְיֵשׁ לָחוּשׁ לִדְבָרָיו לְכַתְּחִלָּה.

 

Seif 9

One who slaughters should take heed and have consideration for the hide so that he does not tear it significantly and slaughtering with the head of the knife so that it is covered by the hide. If one slaughters with the middle of the knife there is no concern about the head of the knife being covered by hide since the knife is in the place of proper slaughter [that is] opposing the signs and not covered. There are those that are strict with this and there is a concern in the matter ab initio.