Our Gemara quotes Rav Naḥman in the name of Rabba bar Avuh who distinguishes between two types of ponds used for washing. He claims that the pond discussed in the Mishna that must be kept three tefaḥim from the boundary is a pond used for soaking the clothing (the maḥmatzan), but the pond of water used for agitating the clothing (the nadyan) must be kept four amot away from the boundary.
The method used for washing clothing – which is still the basic method used today, albeit using a different technique – involved two stages. In the first stage, clothing was left to soak, usually together with soap or some other cleansing agent whose purpose was to break down the oils and other contaminants in the material. The neighborly concerns with this stage of the washing process stemmed mainly from the smells that emanated from the treatment of the dirty clothing.
In the second stage the clothing was moved to a different pond of water where it was rubbed or agitated by hand or by foot, a process which removed the dirt and the cleaning agents from the material. This stage often involved splashing and spraying dirty water, which had potential to damage property well beyond three tefaḥim from the pond.
The Ri Migash offers an alternative explanation for the nadyan, one accepted by the Rambam, as well (see Hilkhot Shekhenim 9:5). He explains it not as a separate pond, but as the rock on which the agitation was performed. Here, too, the potential damage involved dirty water spaying onto the neighbor’s property.