The main reason is to maximise the survival of fish you release. Popular species such as snapper can suffer from barotrauma if caught in water deeper than 10m and the problem can get more pronounced the deeper you fish.
Barotrauma occurs as gases expand inside the fish’s body when it’s pulled towards the surface. The effects and severity of barotrauma injuries increase with depth of capture. Different species are more susceptible than others.
Studies have shown that it’s not possible to prevent barotrauma by retrieving the fish slowly and hoping that it will “readjust”.
How to use?
Select correct size release weight, this would depend on depth water current etc.
Tie the release weights to a hand line with either 3mm cord or 30-40lb or heavier mono fishing line.
Carefully place the fish in the water with release weight attached and let the handline spool free spool down to the depth you caught the fish.
Hook the release weight into the corner of the fishes mouth, hook must be facing down.
Leave it down in the capture depth of the water column until the fish swims off the hook at its own leisure.