Playing on the old law of physics, "opposites attract," the charged gunk molecules "stick" to the bubbles, riding them up the column of water. Once the bubbles reach the surface air they burst, depositing their hitchhikers into a collection cup. This collection cup keeps the accumulated gunk from slipping back down into the water column inside the reaction chamber. Due to the very nature of saltwater, this process is possible. Freshwater protein skimming just isn't feasible at our level as the technology to make it possible just isn't practical at the hobby level.
Bubble size is a fundamental ingredient to a successful protein skimmer and various methods are used to create this "perfect" bubble. Originally, lime wood was and still is used to create the froth. The European hobbyists were amongst the first to recognize the importance of skimming their aquariums. Specifically, the Germans marketed some of the finest models in their day, and still do. Tunze and others brought protein skimming to our shores with the original design. This was called Co-Current skimming.
The basic, Co-Current skimmers used an open-ended tube or cylinder with the bubble source mounted at it's base. As with uplift tubes utilized with under gravel filter plates, Co-Current skimmers used the volume of air bubbles rising in the column to bring the system water into contact within the chamber body. The water was "drawn" up into the cylinder from below the water's surface and once the bubbles burst at the collection cup, the treated or stripped waters simply "fell" back down into the aquarium.
Above is a typical co-current skimmer, either hang-on or sump mounted.