Caroline, the debris and sediment is heavier than water -- it is mostly calcium carbonate, I think. So when you stick a hose into the opening, the water swirls around and carries the particles as the water flows back out of the opening.
This can be a wet and messy job. And the little crystals of sediment can be hard to clean up, if your driveway is either asphalt or cracked concrete (as mine is).
But I don't think you have to worry about the sediment being carried into the rest of your fresh water system. I could be wrong, but I have rinsed my water heater once a year for 16 years and have never had a problem.
By the way, a long tapered brass nozzle makes it easier to get the water into the hole -- a garden hose may not fit.