The Kitzur writes (33:8) that at the time of the tekufa, one should place a piece of metal on all non-cooked, non-pickled, or non-salted, food and drink. I know that this is not followed nowadays, but what is the reason for it?
A time of Tekufa (pl.Tekufos/Tekufot) is defined as time of equinox which occurs four times a year1.
The 1906 Jewish Encyclopedia relates the following regarding superstition at the times of the equinox:
An ancient and widely believed superstition is connected with the teḳufot. All water that may be in the house or stored away in vessels in the first hour of the teḳufah is thrown away in the belief that the water is then poisoned, and if drunk would cause swelling of the body, sickness, and sometimes death. Several reasons are advanced for this. Some say it is because the angels who protect the water change guard at the teḳufah and leave it unwatched for a short time. Others say that Cancer fights with Libra and drops blood into the water. Another authority accounts for the drops of blood in the water at Teḳufat Nisan by pointing out that the waters in Egypt turned to blood at that particular moment. At Teḳufat Tammuz, Moses smote the rock and caused drops of blood to flow from it. At Teḳufat Tishri the knife which Abraham held to slay Isaac dropped blood. At Teḳufat Ṭebet, Jephthah sacrificed his daughter (Abudarham, "Sha'ar ha-Teḳufot," p. 122a, Venice, 1566).
The origin of the superstition can not be traced. Hai Gaon, in the tenth century, in reply to a question as to the prevalence of this custom in the "West" (i.e., west of Babylon), said it was followed only in order that the new season might be begun with a supply of fresh, sweet water. Ibn Ezra ridicules the fear that the teḳufah water will cause swelling, and ascribes the belief to the "gossip of old women" (ib.). Hezekiah da Silva, however, warns his coreligionists to pay no attention to Ibn Ezra's remarks, asserting that in his own times many persons who drank water when the teḳufah occurred fell ill and died in consequence. Da Silva says the principal danger lies in the first teḳufah (Nisan); and a special announcement of its occurrence was made by the beadle of the congregation ("Peri Ḥadash," on Oraḥ Ḥayyim, 428, end 2). The danger lurks only in unused water, not in water that has been boiled or used in salting or pickling. The danger in unused water may be avoided by putting in it a piece of iron or an iron vessel ("Bet Yosef" on the "Ṭur," and Isserles' note to Shulḥan 'Aruk, Oraḥ Ḥayyim, 455, 1; "Be'er Heṭeb," to Yoreh De'ah, 116, 5). R. Jacob Mölln required that a new iron nail should be lowered by means of a string into the water used for baking maẓẓot during the Nisan teḳufah ("Sefer Maharil," p. 6b, ed. Warsaw).
Based on the last statement of the encyclopedia, it seems clear that the Kitzur Shulchan Aruch is basing his ruling on the Poskim cited above to remove the potential dangers of water contained in food and drinks during Tekufa.
Hope this is helpful
1 see here
2 I was unsuccessful in locating this.