This design is a detachable handle you can add to the tip of your cooking knife to cut hard things. I use it for frozen food.
To install it just stick the tip of the knife into the grove and push a little. It will become stuck at the extremity where the groove narrows. You can then securely push on the back to cut hard things. The handle is securely maintained but can be easily removed if you just rotate it around the tip.
This design should be adapted for most knife geometry as long as the blade is no thicker than about 1.8mm. For thicker blades you can scale the model.