It increases the viscosity of the water. Since the operation of a skimmer is dependant on the viscosity, it causes more foaming and more water removal.
I wouldn't use it in tanks myself. There isn't a lot of point to it, and most experienced aquarists will tell you not to dump things into the water just for the hell of it.
It's often used to help fish who are under stress from transport. I'm not against using it for this purpose, but there is no point to use it for water changes or anything like that. If you use proper source water (RODI, properly mixed, aged aerated) and are not doing over 75%, then the fish should be just fine with a water change.
Manufacturers love to tell you to dump their things directly into the tank. They sell WAY more product that way.