There are basically two ways to stabilize a rod-like item (fork, knife, rod, etc) with a prosthetic hook.
The first way to place a knife or other item with a similar shape into the hook is by using the cable attachment as lever to avoid the item to slip out.
While this is the "obvious" way to do it, it is not without its dangers. One can firmly press down - as when cutting a not so tender meat with a not so sharp knife - but watch closely; here, we are still OK with pressing down...:
And due to (a) limited amount of rubbers and (b) the geometry of the knife versus the geometry of the hook device, the knife will swivel under sufficient pressure:
So, for very extensive pressure, I will place the item into the hook's base such as this:
Now, the hook has two claws one of which does NOT move.- Turning that one up as shown here, one can then basically exert very large amounts of pressure onto the item such as a knife.
Adding a small wire loop or rubber band can also help to add stability in any other direction.