Here is what Wiki says:
Flossers are anglers who use the method of "bottom bouncing" to catch fish. Their catch is mainly from the salmon species.
Flossing is a controversial method, regarded by some as an unethical way of harvesting fish. It is also called "bottom bouncing", "snagging" or "ripping". The method employed uses leaders of between 10 and 25 feet (3 and 8 metres) with a 1 to 4 ounce (30 to 115 gram) lead weight called a "Bouncing Betty " (named after an infamously lethal landmine first used during World War 2). Due to angling regulations in British Columbia, Chile, Peru, and Argentina, hooks devoid of any dressing (whether artificial or organic) are illegal.
To work with this method, meat-fishermen often tie on long strands of green or orange wool or Corkies to their hooks. The technique of bottom bouncing is to position the long leader so that it flosses itself through the fish's mouth. The hook attached at the end of the leader then usually pierces the fish's mouth from the outside in as the weight pulls the line downstream. The fish is snagged in the mouth, this is considered by many to be unethical. Bottom bouncing is commonly practiced in the summer months on the Fraser River, when sockeye and chinook salmon run upstream to spawn.
Every year, numerous internet fishing forums are alight with flossing debates as the conflicting sides contest their view points.