The process is called working wet into wet. Wet into wet means the paint medium (be it water colour, acrylic paint, oil paint, ink) is applied to a surface still "wet" with solvent/medium. The solvent used to apply the previous layer of paint is still present on the surface where the new layer is being applied. The solvent has not yet evaporated (or the oil oxidized). With the solvent still present on the surface any additional paint added to the surface will dissolve as it makes contact with the solvent. Depending on the surface the paint is applied to different effects are achieved. Paper, wood panel and canvas are very different surfaces on which paint can be applied. If the solvent is infused into the painting surface (such as the fibre weave of a canvas or paper) then the paint will, through capillary action, be drawn into the surface. Solvent that only sits on the surface (such as when applied to a hard panel board or onto dried paint surface) will only draw the applied paint to where the solvent is present. The wetter the surface (more solvent present) the paint will dissolve further into the surface. But since it's solvent the paint will be further diluted and thinned. Controlling the wetness of the surface controls how far the applied paint is allowed to diffuse and expand from the point of application.

The process is called working wet into wet.
Wet into wet means the paint medium (be it water colour, acrylic paint, oil paint, ink) is applied to a surface still "wet" with solvent/medium. 

The solvent used to apply the previous layer of paint is still present on the surface where the new layer is being applied. The solvent has not yet evaporated (or the oil oxidized). With the solvent still present on the surface any additional paint added to the surface will dissolve as it makes contact with the solvent. 

Depending on the surface the paint is applied to different effects are achieved. Paper, wood panel and canvas are very different surfaces on which paint can be applied. If the solvent is infused into the painting surface (such as the fibre weave of a canvas or paper) then the paint will, through capillary action, be drawn into the surface. 

Solvent that only sits on the surface (such as when applied to a hard panel board or onto dried paint surface) will only draw the applied paint to where the solvent is present. The wetter the surface (more solvent present) the paint will dissolve further into the surface. But since it's solvent the paint will be further diluted and thinned. 

Controlling the wetness of the surface controls how far the applied paint is allowed to diffuse and expand from the point of application.