Shabbos Chapter 10, Mishnah 4. This mishnah continues to discuss the ways in which an object is carried out, and the weight given to a person's intention at the moment of carrying: did the act turn out as the person originally intended?
Two cases in which the object's position changed:
"HaMiskaven l'hotzi l'fanav u'va lo l'acharav - patur" - someone who intended to carry the object in front of him, and in fact it ended up behind him (for example, a pouch that swung around to his back). Something carried behind a person is not guarded as well as something placed in front of him. Since his intention was to carry it where he could keep an eye on it, and in fact it ended up behind him, the act of carrying is not as significant as he intended, and therefore he is exempt.
"L'acharav u'va lo l'fanav - chayav" - in the reverse case, where he intended to carry the object behind him and in the end it came around in front of him, he is liable, since in fact he carried it with better protection than he originally intended.
"Be'emes amru":
The phrase "be'emes amru" ("in truth they said") comes to tell us that this is how reality works, and this is the practical halachah: "HaIshah hachogeres b'sinar, bein milfaneha u'vein mil'acharehah - chayeves" - a woman who is girded with an apron, a sort of undergarment that tends to shift around her as she walks, is liable for carrying whether the object ends up in front of her or behind her.
How does this ruling differ from the previous case, where someone intended to carry in front of him and it came around behind him, and he is exempt?
The reason is stated explicitly in the mishnah: "shekein ra'ui lihyos chozer" - it is the nature of the apron to turn and move around the body, and this is ordinary and expected. Therefore, when she places the object in front of her, she knows that it will eventually come around behind her, and all of this is included in her intention from the outset. It makes no difference, then, where the object ends up, in front of her or behind her, since it is all part of her intention as to how she would carry it out on Shabbos, and so she is liable.
Rabbi Yehudah - the couriers who carried official documents:
Rabbi Yehudah adds that the same law applies to those messengers who would carry the government's written orders. Their practice was to carry them in a kind of tube suspended from a cord or a chain, either around the middle of their body or around their neck, and as they ran and walked the tube would move around them. Since the expectation from the outset is that at times it will be in front of them and at times behind them, all of this is included in their intention, and it makes no difference where it actually ends up: whether in front of them or behind them, they are liable for carrying on Shabbos.
In summary: in this mishnah we learned the weight of intention in carrying: one who intends to carry in front of him and it comes around behind him is exempt, since in fact the object was guarded less well than he intended; whereas one who intends to carry behind him and it comes around in front of him is liable. By contrast, wherever it is the nature of the object to turn and move around the body, as with the woman's apron and the tube of the document couriers, all the possibilities are included in the intention from the outset, and the liability stands whether it ends up in front or behind.