THere's a little more to it to peform this method safely....
First, this method is going to vary from one fish to another, not all fish handle "dips" the same. It's to large of a subject to cover in a forum such as this. If there is a question about a specific fish, post the question and see if we can get an answer.
It is VERY IMPORTANT that the water you "dip" a fish into has the same ph / hardness values as the aquarium water that the fish is kept in. In other words, if your dipping a saltwater angel in fresh water, the freshwater must have the approximate same ph as your marine aquarium. YOu can shock and kill fish very easily placing them in water with radically differant ph values. Temp needs to be close too.
Typically, this method is used to remove parasites from saltwater fish, the radical osmatic differances between salt and fresh will "pop" the parasites attached to the saltwater fish when placed in freshwater. The max I typically dip a saltwater fish in freshwater is about 10 to 15 minutes. It is very normal for the saltwater fish to "curl" it's body or lay on it's side while in the freshwater. THis method is normally not very usefull on anything other than ectoparasites.
Putting a freshwater fish for brief periods of time in water with a higher than typical salt value is a subject of contraversy in terms of how usefull it is against external issues. I'm a bigger fan of extended periods of time that the salt value is raised in freshwater tanks for treating either parasites or external infections. (like 14 days or more) I also have found that daily fluctuation of the salanity during the treatment of freshwater fish is benificial, espcially on infections. Fresh water fish do not drink the water they live in, they "absorb" water thru osmossis into thier tissue, the fluid in thier tissue must match the specific gravity of the water they live in, mostly for boyancy. By changing the the specific gravity daily during treatment periods, the fish's body is litterally purged or "flushed out", this seems to help with curing infection or fungus but I often time combine this method with a proper antibiotic.
I normally perform these types of treatments on freshwater fish in a quarantine system, not the display tank they are kept in.
I have on occasion put freshwater fish in full strenght salt water for 10 or 15 minutes to treat external infections or parasites. Many fish don't deal with that to well. African cichlids (that rarly get parasites) seem to tolerate it real well as do many larger south & central american cichlids assuming that they were kept in freshwater with a higher ph value.
THis is a "in a nutt shell" explaination, there's more to it that listed above but this covers the general "in's & out's" I just wanted to make the point that you can not just simply throw a fish into a dip for up to 30 minutes with no worries. Many fish will die if you don't take the nessessarry precations. I am very experianced with this subject, If I can offer any addition input, I'm happy to share my experiances
Joel