Actually, DM, as you say direct application is more concentrated but we are talking about fluoridated water here. Fluoride exerts its major effect by creating low levels of fluoride ions in saliva and plaque fluid, thus exerting a surface effect. A person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04 mg/L several times during a day.
So quite literally, as my old ma used to say, "just like liquid getting into this chalk".
You don't need to swish it over your teeth (although that will help), you just need to ingest it.

