Reference for Chullin 32:12

מאי מחובר לקרקע סכין דלא מבטל ליה

If, however, the knife was below and the throat of the animal on top, [the slaughtering is invalid], fo it is to be feared that the head might press down heavily upon the knife.<span class="x" onmousemove="('comment',' This would invalidate the slaughtering; v. p. 37, n. 8.');">⁷</span>

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