Flow Maps are used to “disturb” the Sprites UV coordinates. This is done by using a Map that consist of Red and Green colors that represent the U and V coordinates. Sounds technical? Don't worry, for this task we have a handy application called FlowMap Painter.
You can learn in this Video Tutorial how to use FLowMap Painter for creating your own Flow Maps.
FlowMap Painter can be Downloaded here: http://teckartist.com/?page_id=107