Shabbos, Chapter 20, Mishnah 3. The Mishnah deals with three subjects: soaking asafoetida, soaking vetch, and sifting straw.
"Ein shorin es hachiltis b'foshrin":
Chiltis (asafoetida) is a sharp-tasting plant used for medicinal purposes, and at times also as a seasoning. One may not soak it in lukewarm water, and two explanations are offered for this:
Some explain that it appears as though one is preparing a remedy, which is forbidden on Shabbos.
Others explain that this is an activity whose character is that of a weekday.
"Aval nosno l'soch hachometz" - one may place the chiltis, that sharp-tasting plant, into vinegar and dip food into it, in the way people were accustomed to dip bread in vinegar, even though a bit of the medicinal quality of the chiltis enters the vinegar.
"V'ein shorin es hakarshinin v'lo shafsan":
Karshinin are a type of legume used mainly as animal fodder. One may not soak them, because the soaking is done in a manner that brings the waste, the chaff, up to the surface of the water; and likewise one may not rub them, since this too removes the unwanted chaff.
"Aval nosno l'soch hak'varah o l'soch hakalkalah" - one may place them into a sieve, or into a basket that serves as a sieve, even though at times the unwanted parts fall through it. The permission stems from the fact that the separation happens on its own: there is no act of sifting here, only placing them in the utensil that separates, and since one does not actually use the sieve as a sieve, it is permitted.
"V'ein kovrin es hateven bich'varah":
One may not sift the straw with a sieve or a strainer. "V'lo yitnenu al gabei makom gavoha" - and one may also not place it in a high place where the wind blows, "bishvil sheyeired hamotz" - so that the husks of the kernels should separate from it and fall away.
"Aval notlo bich'varah v'nosno l'soch ha'evus" - one may take the straw in a sieve, not in order to sift with it but in order to carry it, and place it into the feeding trough for the animal to eat from, even though in the process some of the waste falls through the holes. Since this is not the manner of sifting but merely the manner of transporting, it is permitted.
In summary: the three halachos in the Mishnah come together around one principle: an act of preparation or separation performed in its usual manner is forbidden; but when the separation happens on its own, incidental to a permitted act in which the sifting utensil is not being used as such, there is no prohibition.