What is a plunging silk?
A sinking line is by definition a line that will sink below the surface, unlike a floating line which will remain on the skin of the water.
There is a plethora of reservoir fishing equipment available, but when and how to use it?
It's not necessarily necessary to have all the silks on the market, but a regular reservoir angler will need a number of silks of different densities to fish at the right depth and offer his flies at the right "stage".


When should you fish with sinkers?
The use of sinking lines allows us to present our flies from a few decimeters below the surface to the bottom.
When the fish are at some distance from the surface, intermediate bristles are the most suitable. There are different densities of intermediate silk in these lines, which are effective from 10 cm below the surface to 1.5 to 2 metres down. Each bristle will be classified by density in cm/second, which tells us something about its ability to sink.
These silks are often neutral, light and/or transparent in color.
Sinking lines allow you to fish between two waters, but also very close to the bottom. They are also classified by density, which you'll find marked on the line spool and on the box, with the letter S standing for sinking
Plunging bristles start at the S3 and extend to the S7. These bristles are often dark, and the darker they are, the faster they sink.

Which plunging bristles to use, depending on conditions?
Midge tip silks with a floating part and a hoover tip are made for fishing just below the surface. Of course, in some cases you can use a floating line and one or more weighted flies to fish in the same layer of water, but they keep your flies in place no matter how fast you retrieve your line. An undeniable advantage for certain types of fishing, such as chiro or washing line fishing.
The long midge tip is a midge tip with a slightly longer front section that will drop a little further.
The slow intermediate line is Rio's Hoover. Unlike the midg-tip, the whole front part (the spindle) flows just below the surface. It therefore stays below the surface with a density of 2.5 cm/s on a 9 m head. It can be used for chiro fishing, but deeper than the midge-tip, and also for streamer or washing line fishing, but in a lower water layer than the midge-tip. It's also very useful in strong winds, for passing just below the surface while maintaining good contact with your flies.
Fast intermediate line, such as Rio's Camolux, will sink slowly (3.8 to 5 cm/sec). If we don't retrieve, the line will flow a little faster than the slow intermediate, allowing us to descend a little further and keep our flies in this layer of water. It's mainly used for streamer fishing, but can also be used for worm fishing, deeper washing line fishing, blob fishing, etc...

The S3 sinking line sinks faster than a fast intermediate and enables us to present our flies more quickly in a certain depth, or even on the bottom in the case of a reservoir or shallow station (2 meters for example). It can be used for streamer, worm or booby fishing. In deep lakes, it can also be used for chiro fishing.
The S5, S6 and S7 sinker lines are used to get to the bottom quickly and especially for booby and bottom fly fishing, but can also be used for streamer fishing in reservoirs or large, deep lakes.
There are also sink tip bristles, which have a floating part and a plunging tip of a certain length depending on brand and model. At Rio, for example, the sink tip is 24 ft (7.30 m).
Different densities of plunging bristles
Here's a diagram showing the evolutionary depth of the different plunging silks, from slow intermediate to very plunging.
The fishing technique used may vary according to the silks employed. Of course, numerous fly options can be used for each technique. These are only suggestions and/or illustrations.

1- Slow intermediate silk (chiro, nymph, drowned, streamer, washing line)
2- Fast intermediate silk (chiro, nymph, drowned, streamer, washing line)
3 - S3 diving silk (chiro, nymph, drowned, streamer, washing line)
4 - S5 diving silk (mainly for boobies/blobbies/fab)
5- S7 diving silk (mainly for boobies/blobbies/fab)
A - Water surface
B - Background
C - Depth