Summary

Yosef Karo Shulchan Aruch — Yoreh Deah 18 The law of checking the knife and its flaw

If perhaps he reversed the direction, the slaughter is unacceptable. And one who cuts and doesn't feel the flaw before the slaughter, only if we found his knife acceptable before the slaughter; however (now) after the slaughter we find a disputable flaw, as it is said: it is clear me that if he cut in only one direction, the direction without flaw alone, the slaughter is unacceptable.
סכין שתבדק בהולכה, ולא הרגיש שיש בה פגם, וכשהחזיר אותה בהבאה הרגיש שיש בה פגם – זו הנקראת "מסוכסכת", אם שחט בה דרך הולכה ולא הביא – שחיטתו כשרה.
Source: Wikisource

Yosef Karo Shulchan Aruch — Yoreh Deah 18 The law of checking the knife and its flaw

There is need for a lot of clarity of mind and fear of heaven in checking the knife, If this is not apparent look that he checks it 3 times without feeling the damage for one minute, afterwards we find what he slaughtered recently, we test him to get a sense and feel in accordance with the intention of his heart.
Source: Wikisource

Yosef Karo Shulchan Aruch — Yoreh Deah 18 The law of checking the knife and its flaw

Because the flaws are many we worry when he returns his hand and kills (he will) not feel it.
סכין שיש בה פגימות הרבה: אפילו כולן מסוכסכות – אפילו שחט בה בדיעבד פסולה, דמתוך שפגמותיה מרובות – חיישינן שמא השיב ידו מעט ולא הרגיש.
6 When a knife has been sharpened, but its edge is not smooth, instead, when one touches it, it is like touching the tip of an ear of grain which becomes stuck on one's finger, [nevertheless,] since it does not have a blemish, one may slaughter with it.
Source: Wikisource

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