How I take the bead for a walk. Ensure board is tilted 30 degrees. I use a DaVinci Squirrel mop brush as it holds huge amounts of pigment!

It's about "the bead." The bead is what internationally renowned watercolour artist Joseph Zbukvic calls the wet edge of your painting. 

Kandiah, Perceival Selvanathan (2011 p9.) provides a useful response to this question. See below in italics: