יורה דעה ז׳Yoreh De'ah 7

Se’if 1א׳

יָכוֹל אָדָם לִקְבֹּעַ סַכִּין בְּגַלְגַּל שֶׁל אֶבֶן אוֹ שֶׁל עֵץ, וּמְסַבֵּב הַגַּלְגַּל בְּיָדוֹ אוֹ בְּרַגְלוֹ, וּמֵשִׂים שָׁם צַוַּאר הַבְּהֵמָה אוֹ הָעוֹף עַד שֶׁיִּשְׁחֹט בִּסְבִיבַת הַגַּלְגַּל; וְאִם הַמַּיִם הֵם הַמְסַבְּבִים אֶת הַגַּלְגַּל וְשָׂם הַצַּוָּאר כְּנֶגְדּוֹ בְּשָׁעָה שֶׁסִבֵּב וְנִשְׁחַט, הֲרֵי זוֹ פְּסוּלָה, (ל' הַרַמְבַּ"ם שָׁם דִּין י"ג) וְאִם פָּטַר אָדָם אֶת הַמַּיִם עַד שֶׁבָּאוּ וְסִבְּבוּ אֶת הַגַּלְגַּל, וְשָׁחַט בִּסְבִיבָתוֹ, הֲרֵי זֶה כְּשֵׁרָה בְּדִיעֲבַד, שֶׁהֲרֵי מִכֹּחַ אָדָם בָּא; בַּמֶּה דְּבָרִים אֲמוּרִים, בִּסְבִיבָה רִאשׁוֹנָה שֶׁהִיא מִכֹּחַ הָאָדָם, אֲבָל מִסְבִיבָה שְׁנִיָּה וּלְאַחֲרֶיהָ, פְּסוּלָה שֶׁהֲרֵי אֵינָהּ מִכֹּחַ הָאָדָם, אֶלָּא מִכֹּחַ הַמַּיִם בְּהִלּוּכָן.

A man may fix a knife in a wheel of stone or of wood, and turn the wheel with his hand or with his foot, and place there the neck of the animal or of the bird until he slaughters by the turning of the wheel. And if it is the water that turns the wheel, and he placed the neck against it at the hour when it turned and it was slaughtered, this is invalid. (the language of the Rambam there, law 13) And if a man let loose the water so that it came and turned the wheel, and he slaughtered by the turning of it, this is valid after the fact, for it comes by a man's force. In what case is this said? In the first turn, which is by the man's force; but from the second turn and afterwards it is invalid, for it is not by the man's force but by the force of the water in its flowing.