Yosef Karo

“ Seif 2 One who shechts with something connected to the ground or a body, for instance nails or teeth connected to a beast, his shechita is invalid. If something used to be detached from ground and was ultimately attached to earth, he may not shecht. ”

“ A knife that has one edge blunt and one edge fine, he initially should not shecht with the fine side. This is a gezeirah [protective restriction] lest he shecht with the other side. And if he shechted, and he did use the fine side, his shechita is kosher.
Rema: The same is the rule for a long knife that has a blemish, and is left with a long enough section without blemish for shechita. It's forbidden to shecht with it, even with the good part, even if he wrapped a cloth over the blemish (Tur.) ”

“ That's when the beast's neck is below and the knife above, for if the beast's neck was above the knife, we fear lest the beast bears down with its body's weight without to and fro action, and that's not shechita. ”