A couple of years ago, I was first introduced to yuzucha (citron tea), when my host-sister gave me a bottle of Korean yuzucha. I didn't know much about it then, but after trying it as a drink and as a topping on yogurt and also on toast, I was hooked!
This yuzucha is made by a company in Kochi prefecture on the island of Shikoku--the same company that makes a yuzu drink that I like called Gokkun.
I think yuzu and other citrus fruits, such as mikan (tangerine) have the power to keep you warm, that is why it is added to onsen or furo(hot spring or bath water)--plus, it smells nice and refreshing.
Yuzucha has no actual tea in it, in fact, it looks like runny marmalade.
Just put a tablespoon or two (depending on how sweet you want your drink) into your teacup and add hot water.
It is a nice way to warm up and I also love eating the strips of yuzu at the end.