Welcome to the Daily Halacha (Jewish law). We all know that a person on Shabbat cannot squeeze a sponge or squeeze the water out of a cloth. It's called sechita. Squeezing on Shabbat is not allowed. There's another form of issur (not allowed) that maybe some people are not aware of, and that is sechitat se'ar, which means a person cannot squeeze hair on Shabbat. It's midrabanan (rabbinic), it's issur midrabanan, not min haTorah (from the Torah).
What does sechitat se'ar mean? Let's say a person goes to the mikveh (ritual bath) on Shabbat. He's one of those who has the minhag—he has a tradition of going to the mikveh on Shabbat. And now his hair on his head is wet, and his beard is very wet. Halachically, he's not allowed to squeeze it out. What he could do, says the Ben Ish Chai, is take a very, very thick towel, a towel made of very thick material, and dab it on his head and on his beard. But to outright squeeze water from the hair or beard is issur midrabanan.
This happens sometimes with teenagers. Teenagers get their hair wet. They're trying to make their hair on Shabbat, so they get it very, very, very wet. And then they brush their hair and try to squeeze the water out of their hair. Got to be very careful. That's issur midrabanan. If you want, you can sprinkle some water on your hair and then brush it, but to put so much water on your hair that you're going to have to squeeze out the water is issur midrabanan on Shabbat.
This also applies to girls. If girls want to make their hair, their long hair, on Shabbat, you're not allowed to wet it. Now you're going to have to squeeze out the water. What you could do is take a brush, a special brush for Shabbat, because that's what we do. We have a special brush just for Shabbat. Make sure that the actual sticks of the brush are far away from each other, so that when you use it on Shabbat, it's not going to automatically pull out your long hair when you brush it, because then it's also going to be issur. So make sure it's one of those brushes that does not pluck out hair when you brush your hair. You take that brush, wet the brush or your hair a little bit, sprinkle some water, and then start brushing your hair. But to actually get your hair wet, start brushing it and making your hair, and then squeeze water out of your hair on Shabbat, is issur midrabanan; it's not allowed on Shabbat.