What kind of touching is the Torah referring to?
Rashi: It is not touching from the outside, for this is not Metamei a K'li Cheres. Rather, it is moving it - this is like touching the entire Kli 1 .
This only applies however, provided the opening of the K'li is wide enough for the Zav to place his finger inside it - to preclude earthenware jars with a narrow opening from the law of Heset (Rashi, Shabbos, 84b). See also Sifsei Chachamim and Ba'al ha'Turim.