Fluid control question

I hope this is the right place to post this.
OK. So I’ve downloaded the build used in these http://www.youtube.com/watch?v=WruTNnF6Ztg fluid tests and all I wan to know is what the velocity option in the control tab does. I know what the attraction does but whether I put the velocity options as high or as low as possible I don’t see any difference:spin:. Am I missing something really obvious? What does the velocity options do?