same for what it does for your skin, nails, whatever. it closes them. when you heat up that stuff you’re expanding it, allowing more things like oil, dirt, products, etc to soak in.
so like when you get your hair colored they heat it up and it goes faster. and hot oil hair treatments. and uhhh steaming your face, soaking your nails in hot water (that’s more for your cuticles tho but i mean u get it)
that hot water makes your stuff more porous, being porous goes both ways. it lets things in and out much easier. getting it cold shuts it right back up so it’s not as porous anymore. that way it locks the moisture back in your hair, closes the cuticle layer of the hair strand so moisture (and colour) can’t escape as easily.
i explain stuff poorly and long handed- let me explain better
when you wash your hair with hot water it opens the cuticle layer of your hair, allowing things like conditioning products in and color out. when you get out of the shower this way your cuticle layer is still open, which means more porous hair that takes longer to dry, and dries frizzier.
when you blast your hair with cold water not only will your color last long, your hair will be less frizzy when it dries, but it will also trap in that moisture so more stays instead of being on the outside and wiped off with a towel.
also it feels fucking great and is great for your scalp for all the same reasons and holy fuck start doing it every time you go to a hair salon because yeah it sucks when you’re in a shower but at a salon it feels like an all over body awakening like it’s so fucking amazing.