Bungee is used for numerous things but two of the most common in spearfishing are "breakaway systems and dive knife tethers.
A "breakaway" system is a method of hooking your shaft and shooting line directly to your float line so that when you shoot a fish it "breaks away" from your gun. (See Image)
You tie a small length of bungee from the end of your shooting line to the shooting line release clip on your speargun.
The other common use for bungee is to hook to your knife handle and loop under your knife sheath.
Available in (2.9mm red), and (3.2mm orange) which is better for breakaway system and 5mm which is better to hold your knife
*Sold by the foot