In the gemara's discussion , the gemmara darshens the posuk "(lishmor meod)v'laasos" to teach that one may wear a siv on his leg or carry a pole on his shoulder even though it may cause the tzaraas lesion to fall off. The gemara discusses this in light of the machlokes R Yehudah and R Shimon and davar sheno miskaven/ psik reisha. Isn't it really an issue of misasek, and therefore why is a drasha necessary at all?
gary schreiber, chicago, il
Despite the fact that there are varying opinions as to the definition of Misasek (i.e. see argument between Rashi and Tosfos 72b), everyone agrees that the case of Misasek is when a person does not have any prior awareness that he would be doing a specific action to the specific object which he ended up doing that action. The only exception to this rule is when someone thought that the object that he was performing the action upon had a different status (i.e. Talush vs. Mechubar), which regarding Misasek changes the item's status as if it were a totally different item.
However, in the case brought in the Gemara, a person knows that if he puts a pole in a certain area where he has a lesion, that it is very possible that the lesion will fall off when he is using that pole. This is not called Misasek, as he has prior knowledge that his lesion may be torn off as a direct result of placing the pole in that area.
Kol Tuv,
Yaakov Montrose