In light of the comment here, that
Koseiv involves creating any meaningful letter or symbol. It doesn’t have to be with ink on paper. A rubber stamp, carving into stone or cutting a letter out of paper all involve koseiv. Even forming a letter out of something temporary – such as those toys where one uses a magnet to manipulate iron filings – is prohibited on Shabbos under this melacha. Koseiv applies to letters or symbols in any language, including Braille, Morse code and the like.
The Biblical prohibition only applies to permanent writing. Temporary writing is prohibited rabbinically. So breathing on a window to fog it up then writing on it with one’s finger (for example) is prohibited at the rabbinic level.
--why is it permissible, as I know it is, to make your hands into letters on Shabbat?
In duchening, which I understand happens most Shabbosos in Israel, and which is obviously allowed, the kohanim are deliberately creating a (very) meaningful letter. I just don't see why there is any serious difference between arranging your hands in the shape of a letter and arranging something else--say, iron filings--in the shape of a letter. Both are temporary; both are achieved through media that are not*(?)* muktzeh; both are very easily reversible. And yet one is a melacha and the other is not.
I know that duchening with the gestures would anyway be permissible because it is done l'shem mitzvah, but is it or is it not a melacha? If not, why not?
Note: My question is based on the assumption that the Kohanic hand sign is intended to resemble a 'shin' for Shakkai. I have heard this repeatedly, but could not find a reliable source indicating as much. In case I am wrong about the relevance of duchening, please instead consider the general case of making letters with one's hands on Shabbos--or, for example, this.
