Kommentar zu Wajikra 15:12

וכלי חרס אשר יגע בו הזב AND THE VESSEL OF EARTH THAT HE WHO HATH THE ISSUE TOUCHETH [SHALL BE BROKEN] — One might think that the law applies also if he touched it from outside (its exterior) etc., as it is stated in Torath Cohanim (Sifra, Metzora Parashat Zavim, Section 3 1).

This is [when one] causes it to move. Meaning: Its movement to and fro is caused entirely by the zov’s power, because there is no [such thing as] partially moving, only entirely. The reason Scripture uses the expression “touching” for “moving” is to say that touching is compared to moving — just as touching is [accomplished] with his hands from outside, so too, moving is [accomplished] with his hands from outside — to exclude [the case of] a reed [held] in the zov’s body folds. If there was a reed in his body folds and he moved a pure [object with it], it is pure, since the moving is caused only by his hidden parts.

וכלי חרש אשר יגע בו, “and any earthen vessel which the person afflicted with involuntary issue touches will have to be broken;” in this verse the Torah uses the pronoun בו, implying touch from the outside, whereas in Leviticus 6,21 where the Torah wrote about holy things within an earthen vessel it stated that that vessel must be broken, as it cannot be used for a profane purpose anymore, some of the holy stuff having been absorbed by it, the same pronoun is used, instead of בתוכו, “inside it.” From this we derive, seeing that we have stated that the zav contaminates only an airspace, that if here the Torah used the word בו, it must mean that even indirect touching confers ritual impurity here. This impurity is called tumat hesset, and in practice it means even indirect contact such as sitting on top of several blankets confers this impurity even to the lowest blanket in that stack, or even shaking something so that it will come into contact with the source of the impurity. (Sifra)