Necessity does it again. Chihiro Tanaka wanted to do something and what he wanted didn't exist. So, he improvised. Excerpt from the very imformative Russel Stutler website:
Chihiro Tanaka, the man who is credited with inventing these. He used to empty the ink out of regular brush pens and fill them with water. Then he included photos of these modified tools in his books on watercolor sketching. Some of brush pen makers realized he was on to something big and started to produce these brush pens with translucent barrels and no ink, calling them waterbrushes (mizu-fude)
A proper demo of the brush from da man himself.
Picture of the man.
Page of thumbnails of How-to books on the waterbrush. All in Japanese unfortunately.