Bungee line tensioner
 
The line tensioner was created to facilitate the insertion of the line into the rifle coupling even with gloves, but it can give other advantages.
 
Cushions the stroke end of the shot reducing the wear of the monofilament on the shaft.
 
Cushion the jerks of the caught fish - limiting the risk of losing it.
 
Avoid line vibrations on the speargun tube, which frighten the fish.