This is what the DSLR manual will call "more advanced techniques".
Well, I don't know anything about that. But, it's tricky, and I'm still learning it.
It's a setting on the DSLR (Canon Rebel XS) called "Tv".
Generally, the manual instructs users to try this with running water, like water fountains and waterfalls, and I often see this effect on running water to give it a foamy look. But, the manual says it can be used for motion blurs on stuff like running children, or whatever.
The trouble with it is that it has a tendency to white wash everything, especially in direct sunlight. So, I haven't quite figured it out entirely when programming the camera. My only trickity-lil-trick so far is to just shoot in ambient daylight, like cloudy or partly cloudy when a cloud blocks the sun. Unless I figure out a better way to program it. (Still learning.)
I still haven't tried it at night yet, either. But, I would like to. So, these are all just experimental for me.
To give you an example of just what exactly this is, I'll show a comparison: