Flooding a surface is like taking a huge brush and applying its settings to all the vertices on the surface. When you flood a surface, the weight of each vertex in the node changes according to the value and operation set for the tool.
To flood a node, follow the steps under To paint attributes, but instead of painting in step 7, click the Flood button or use the hotkey Alt-f (Windows and Linux) or Option-f (Mac OS X).
