The melakha of Libun is cleaning wool or linen and whitening it before making it into clothes. Kibus, which refers to washing clothes, is a tolada of this melakha (MT 9:10-11).
There are three stages of Kibus: soaking, scrubbing, and wringing. Since in each stage some of the dirt in the clothing is removed, each stage is prohibited by Torah law. We will now describe these stages in detail.
First, Kibus is done by soaking an item of clothing in water. Soaking causes stains on the clothing to become lighter, and some of the dirt absorbed in the clothing is removed and transferred to the water. Therefore, it is prohibited by Torah law to soak dirty clothes in water. For example, one may not leave dirty baby clothes in water, even though he intends to do the primary washing after Shabbat, since the soaking begins the cleaning process.
The second stage is scrubbing the clothing while it is still wet. This is the main phase of Kibus, because through the scrubbing, the dirt that is stuck to the clothing is removed, disappearing into the water.
The third stage is wringing out the clothing and removing the water it absorbed. When the water is wrung out of clothing, the filth that was transferred to the water is removed as well. Since a little bit of dirt comes out with each wringing, it is prohibited by Torah law. Even if clothing got wet in the rain, one may not dry it by wringing it out, since this will definitely clean it somewhat as well. To ensure that one will not end up wringing anything out, the Sages forbade picking up clothing that is soaking wet. Despite this, one may continue wearing clothes that got wet in the rain. This permission even extends to a case where one had taken off the wet clothes; if he has nothing else to wear, he may put them back on. However, one may not move them around for no reason (SA 301:45-46).
Sometimes there is a fourth stage – drying the clothes with heat. After wringing out the wet clothes, they are often placed on or near a heat source to dry. This evaporates the remaining moisture together with any remaining dirt, leaving the clothing clean and bright. This used to be part of the way that raw wool was processed. After it was sheared and washed, it was whitened in an oven. This action is called Libun and is prohibited by Torah law. Therefore, one may not place a wet coat or a wet towel next to a heater if the temperature will reach yad soledet bo (SA 301:46; see above 10:4).
It is important to realize that it is not just laundering an entire garment that is forbidden. Even removing a single stain is prohibited by Torah law. This prohibition applies whether the stain is removed with water, spit, or any other cleaning agent, including kerosene or benzene. Similarly, if a greasy substance fell onto a garment on Shabbat, one may not spread talcum powder on it to prevent staining (SSK 15:27).
The Sages also prohibited things that might cause others to think that one violated a Torah prohibition and washed clothing on Shabbat. Therefore, clothes that got wet on Shabbat may not be hung on a clothesline or the like. Rather, they should be hung somewhere that people do not generally hang clothing after it is laundered, such as over a chair or on a hanger. Nevertheless, clothes that were hung to dry before Shabbat may be left on the line over Shabbat (SA 301:45).