Is sechita of the hair on shabbat a Rabinic prohibition? Do many Poskim hold thus?
Sechitah of the hair is a somewhat complex issue.
The Gemara writes that there is no sechitah concerning hair (Shabbos 128b), yet, there remains a rabbinic prohibition for somebody who squeezes out liquid for the purpose of collecting the liquid.
Usually, when we dry our hair or squeeze out the water in it, we do not mean to collect the liquid, and therefore some rishonim write that there is no prohibition of drying one’s hair on Shabbos. This is also implied by the Mishnah in Yoma, which states that the Kohel Gadol emerged from the mikva, and proceeded to dry himself with a towel. No exception is made for the hair.
However, although the Beis Yosef (Yoreh De’ah 199) appears to lean towards the lenient opinions, the Mishnah Berurah (326) is stringent (following the opinion of the Magen Avraham and the Vilna Gaon), and therefore one should be careful not to squeeze out water or to dry one’s hair with a towel on Shabbos.
The reason for this prohibition is that although there is no prohibition of sechita per se, there is a prohibition of mechzi ke-sochet (based on the Ran; the Rashba is lenient) — squeezing out water from one’s hair “looks like sechita,” and it is therefore the subject of a rabbinic prohibition.
Rabbi Shlomo Zalman Auerbach has ruled that there is no prohibition in placing a towel over one’s wet argued, and gently dabbing. Doing so will not fall under the category of “looking like sechita.”