Flossing is getting your line to drift into a salmons open mouth. As the line (weight) continues to drift downstream it will pull the line below the weight across the fish' s mouth until the hook embeds in the fish's jaw. When the fisherman keeps his line at a low angle to the water, and "swings" his fly across the current it's quite obvious to see one employing this technique.
In especially deep water, the line enters the water almost perpendicular, the weight sinks to near bottom and the fly below the line just twirls around in the current as it drifts downstream. The line between the weight and fly will occasionally drift into an open mouth and as it continues to drift down stream, will pull the hook into the side of fishs jaw.
If one is fishing a good holding lie, there could be a dozen or more fish there at a given time. That's a lot of open mouths, and catching a fish is almost a given. In fact, if you get the right drift, it's as close to guaranteed as you can get.
There is an art to it and some folks are especially adept at it. It is what it is and my guess is that 85% of the fish caught on the SR are caught this way, whether intended or not. If the hook is set on the fish's far side jaw, chances are very good he was flossed. Again intentionally or not.