Kayran trap is specifically designed for use on a kayran. There are three glowing icons evenly spaced around the kayran, any of these can be used to place the kayran trap. Then a 
appears onscreen when Geralt can activate the trap. When activated, it immediately cuts off the tentacle that activates it, saving the player the trouble of cutting it manually.
If the trap is not used effectively, it cannot later be shed from inventory. It cannot be sold nor dropped.