The Mishna teaches that someone who rents an ox to plow in the mountains but uses it to plow in the valley will not be held liable if the kankan breaks, since the valley is less rocky than the mountains. If, however, the agreement was to use the animal in the valley, but it was taken to plow in the mountains, then the renter would be responsible for damage done to the kankan.
Although there is a minority opinion that the kankan is the yoke on the neck of the oxen, Rashi and most of the commentaries interpret it to be the blade of the plow (the plowshare).
The Gemara recognizes that the responsibility of the renter only applies if he used the animal in a manner that they had not agreed upon. The question raised by the Gemara is who would be responsible in other cases, for example, if the renter did not diverge from the agreement but the workers had broken the plow? In this case, the Gemara brings a difference of opinion –
While the halakha follows Rav Sheisha’s opinion, if it is a particularly rocky area, they will both share the responsibility.
It is clear that the person holding the parasha (the goad) is the one leading the animal – in Aramaic the parasha is the person who “teaches” or directs the animal – oftentimes with a stick in his hand walking behind the animal, while simultaneously holding the handle of the plow (the mana). Our Gemara is describing a situation where there was a pair of oxen, or, perhaps, where the ground was particularly hard and rocky. In such cases two people worked with the animals – one who directed the animals to stay in line with the trough in the field and the other who kept the plow safely in the ground, avoiding rocks.