These are designed for four to six weight fly lines. This removable sinking line sections turns your floating line into an "instant sink tip", eliminating the need to carry a bulky extra spool with sink tip line. The sinking section connects to the line and the leader butt via a loop-to-loop connection.
Sinks at approximately 3.5" per second.

Fishing Tip from Harry Murray
When the streams are high a good technique to enable you to get your nymphs and streamers very deep if you not have a sinking tip line is to use an Instant Sink Tip four foot system. By installing this into your leader the way I show on page 20 of my book Fly Fishing Techniques for Smallmouth Bass you will be able to swim your flies right along the stream bottom.

See my book for the best method to install and fish these.