הלכה על חולין 17:41

And behold I will inform you of the distinction between them in their leather and in their sewing: Know that we make the leather, in which we store [the parchments of] the tefillin of the arm, one chamber; and we place there the four sections that we mentioned, written upon one parchment. But the leather of that of the head we divide into four chambers; and in each one, we put one section from these four sections. And while it is still moist, we make from the leather the resemblance of a [letter,] shin with three heads on the right [side] of the one that is laying the tefillin; and the resemblance of a [letter,] shin with four heads on the left [side] of the one that is laying [the tefillin]. And we insert straps for the head into the extra leather at the end of the chamber, and that is what the Sages called, maaboret, meaning to say that we pass (maavirin) the straps through it. And afterwards we encircle the measurement of the head of the one that lays [them] with the straps, and we make in that place [at the bottom of the back of the head] one knot in the shape of a [letter,] dalet. And it is impossible to describe this in writing, but everyone teaches his students and it is well-known among us, the holy people. And this is the knot of the tefillin that they, may their memory be blessed, said (Chullin 9a) that every Torah scholar needs to know. And they enumerated it with other things that they said about them that it is fitting to know [how to do] them no matter what. And they are the writing [of the letters], slaughter, circumcision, the blessing of bridegrooms and tsitsit. And the length of the strap of the head is enough to encircle the head with it and tie the knot behind him and that there be enough of it to stretch the two ends of the strap until [they are] across from his navel - or at the very least, that one end is until it is across from his heart and one is until it is across from his navel. And the place of the placement of the tefillin of the head - meaning to say the box of leather that the sections [of the Torah] are placed in - is across from the brain, the place where the brain of a baby is frail (Menachot 37a). And this is the explanation that has been received by us about "and they shall be as totafot between your eyes" - that this is what is called 'between the eyes.' And behold, the one who places it actually between his eyes contradicts the words of the received tradition. And the rest of the details of the commandment and all of its content is like its fellow that preceded [it].

From the laws of the commandment are that which they, may their memory be blessed, said (Chullin 9a) that five things spoil the slaughter, if one of them happened during the slaughter. And these are them: pausing; pressing; submerging; sliding; and tearing. The content of pausing is, for example, that he began to slaughter the esophagus, and before he slaughtered its majority, he interrupted [the] slaughtering. If he paused with this interruption the measure of enough [time] for another slaughter, his slaughter is disqualified. The understanding of another slaughter is the [time needed to] slaughter the skin and the benchmarks of another beast like it, and like that of small beast (lamb) for a chicken. And there are those that are stringent, that the measure is only enough for the slaughter of the majority of the [two] benchmarks with a beast, and the majority of one benchmark with a bird. But if he slaughters with a bad (meaning dull) knife, even if he goes back and forth the whole day, his slaughter is fit - except if he brings it back and forth after he slaughtered the majority of only one benchmark in a beast; as if he brings a bad knife back and forth on the minority that remains of it like the measure of pausing, his slaughter is disqualified.