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Research question!
What is the best and sexiest updo hairstyle for really long hair when you need to undo it with one movement and let all the hair just flow down like a shampoo commercial?
@SDZ not sure what you‘d call it, a French Roll perhaps? Twisted into a sort of roll / knot on the back of the head and held with a single chopstick or a pencil or a decorative pin?
@antinomy Oh! That looks perfect! Very elegant, then pull out the chopstick and whoosh! Thank you so much!
@SDZ I've always heard it called the French twist, not the French roll. The ideal length for that hairstyle is the length in that video; you can do it with longer or shorter hair, but it can take a lot more skill. And there are different methods, though I think all of them will come down fast. The preparation of the hair might be important. Well-moisturised hair seems most likely to do what you want: conditioner, mask, toner, oil can all help.
@yingtai That's another thing I was considering. My character has long hair, so I assume it wouldn't really drop like that without being, like oiled up or something? But this is the magic of fiction, I suppose. :)
While I've got you hair experts' attention. I'm looking for another word - for the hair that comes down in a strand over the sideburns. In Chinese it's something like 长鬓发. Best I can find in English is just "a strand of hair that comes down over sideburns." But that ain't cool.
@SDZ wisp, maybe? Sorry I can't think of a better term!
@yingtai English has too many words, but also, English needs more words! 
@SDZ by the way, I think I was wrong about the hair needing to be very moisturised in order to fall down like that. I think it just has to be not wet. I didn't realise, because lately I only do French twists while my hair is wet!