When buying Sisit, a person should know what to ask for. Some features are integral to the Misva and some are Humrot-stringencies to enhance the Misva.
The Shulhan Aruch clearly states that the strings must be spun L'shma-for the sake of the Misva. Therefore, strings manufactured by non-Jews, even if a Jew oversees the process, are invalid. However, the stages before the spinning, including the "Niputz"-the combing of the wool, do not require L'shma, according to most early authorities. However, the MaHaram M'Rotenberg did require that even the Niputz be done L'shma. Therefore, if possible, one should try to obtain strings that meet the stricter standards of the MaHaram. Such strings are available today in stores for a few extra dollars.
There is a controversy whether the Sisit may be spun by machine. Hacham Bension held that such strings are invalid, and therefore one should make an effort to obtain Sisit spun by hand.
The next stage in making Sisit is "Shezira"-twisting several thin strings into a single cord of Sisit. If you look closely at your Sisit, you will notice that each string is actually comprised of several thinner strings twisted together. The Rambam holds that Shezira is optional. However, Maran rules in accordance with the majority opinion that Shezira is mandatory, and it must be done L'shma.
Minimally, Shezira requires only two threads be twisted into one. However, The Ramah MiPano, as well as the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) in Parashat Noach rule that it is best to have each cord made of eight thinner threads twisted together. This results in a total of 256 individual strings in every Tallit: 8 thin strings comprising each of the eight strings for each of the four corners. This corresponds to a certain mystical intention alluding to the wings of the angels, as elaborated by the Ben Ish Hai.
Sometimes the strings that are twisted together become unraveled. Technically, this is not a problem as long as four centimeters remain. In order to prevent this, some have the custom to tie a little knot at the end of each string. This was the practice of the Rabbenu HaAri (Rav Yishak Luria of Safed, 1534-1572), and is cited by the Rama. However, today this is usually not necessary as our strings are generally twisted very tightly and rarely become unraveled. There is a disagreement amongst contemporary Poskim whether one may apply a bit of glue to the tip of the string to prevent it from becoming unraveled. Hacham David, in his Halacha Berura, permits it, whereas Rav Wosner, in his Shebet HaLevi, opposes this, since this was not the traditional practice in previous generations.
SUMMARY
The ideal Sisit should be combed and spun L'shma and be twisted with 8 thin strings.
