Opening the hide of game from the inside out. A gut hook is a small sharpened hook ground into the spine of a hunting blade near the tip. You slip the point under the hide and draw the hook along, cutting the skin from beneath without pushing the edge down into the muscle or organs.
The hook traps the hide in its notch and cuts it as you pull. Because the cutting happens inside the notch rather than at the main edge, there is no way for the blade to sink into what is underneath. That is the whole point of the feature.
It sits on the spine, so it does not interfere with the main edge and the knife works normally the rest of the time.
No flat stone reaches inside a gut hook. It needs a small round file, a tapered ceramic rod or a purpose-made gut-hook sharpener.
A dull gut hook tears rather than cuts, which is worse than not having one. If you buy a knife with one, buy the rod at the same time.
Many hunters do the same cut with the tip of an ordinary blade, two fingers guiding it under the hide, and never miss the feature. It is a convenience, not a requirement.
The hook can also catch on a sheath, so look for a sheath cut to clear it.