Soba Yu is the water that soba noodles have simmered in.
When a customer in a restaurant has finished eating his soba noodles, it's brought to the table in a small pot.
You pour it into the remaining broth in your noodle bowl to make a "final course" soup.
The simmering water is considered to contain nutritious elements from the noodles that dissolved into the water.
"Yu" means "water" in Japanese.
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