[If] he severed the head with one stroke, it is invalid. [If] he was slaughtering and severed the head with one stroke: if the knife measures the width of one neck, it is valid. [If] he was slaughtering and severed two heads with one stroke: if the knife measures the width of one neck, it is valid. When does this ruling apply? [Only] when he made a forward stroke but not a backward stroke, or if he made a backward stroke but not a forward stroke; but if he made a forward stroke and a backward stroke, however short it was, even with a scalpel, it is valid. [If] a knife fell and slaughtered, even though it slaughtered in the normal fashion, it is invalid, as it is stated: And you shall slaughter . . . and you shall eat (Deut. 12:21) — what you slaughter, you may eat. [If] the knife fell and he picked it up, [or if] his clothes fell and he picked them up, [or if] he sharpened the knife and became fatigued and his companion came and slaughtered — if he paused as long as it takes to slaughter, it is invalid. R’ Shimon says: If he paused as long as it takes to examine.