Fast sinkers enable fast working of a fly in deep water and ensure that no matter how fast the retrieve, the lure will be unlikely to rise above midwater. Obviously they are well suited to the fast-flowing rivers where the trout or salmon are taking fairly deep.
Very fast sinking lines
Very fast sinkers are fairly specialized lines, often carrying lead in the core. By their nature they are ideal for the very deepest reservoirs, lakes and lochs where the quarry is feeding deep, and where it is impor-tant that a lure stripped quickly does not rise much above the bottom. Similarly, they are ideal for the biggest, fastest-flowing rivers, enabling the angler to remain in close touch with his fly throughout every cast. In a big Stillwater where the bed snags the fly regularly on slower retrieves, it is a useful dodge to shorten the leader, and attach a dressing incorporating buoyancy material, such as Ethafoam, so that the fly floats up off the bottom clear of the snags, allowing retrieval to be slowed right down. –