In Hilchot Shabbat, there is a prohibition against wringing out wet clothing, as it is part of melabein – the laundering process. Similarly, according to the Gemara, one may not drench their clothing because of the fear that one will wind up wringing it out. There’s another Gemara, however, which seems to contradict the first one. It records that one may ford a river on Shabbat in order to greet one’s mentor.
The Ashkenazic Rishonim offer three different ways to harmonize the two Gemaras. The Rashbam says that presumably the clothing that one is wearing is clean. There is no melacha involved in laundering clean clothing since it is a completely superfluous act. The underlying rationale is that the Torah forbids ‘melechet machshevet’ – meaning that the prohibited act must accomplish something positive or constructive. Laundering clean clothing accomplishes nothing.
Rabbeinu Tam says that fording a river is ‘derech lichluch’. One does not ford a river in order to get his clothes clean; on the contrary, fording a river is a good way to soil your clothes. This is similar to Rashbam in that he sees this act as inherently different from the standard act of libun, but different in that he sees it as a fundamentally different act. Getting your clothes dirty cannot be conceptualized as a form of doing laundry, no matter how you slice it.
The final position is that of the Sefer Yereim (R’ Eliezer of Metz). He accepts neither of these answers and in fact maintains that under normal circumstances, fording a river is prohibited on Shabbat. But what about our student who is traveling to see his mentor? He was wearing clothing which is not laundered in water, i.e., leather.
I remembered this position as the ‘Hell’s Angels’ Yereim’, because of that image of the completely leather-clad dude, and that’s why I remember the sugya these several years later.