A finger choil is a rounded cutout in the blade just ahead of the handle, sized so your index finger can sit inside it. Choking up into it shortens the working blade and moves your hand directly behind the edge for close, controlled cutting.
Grip the handle normally and you have the full blade. Slide your index finger forward into the choil and the pivot point of the whole knife moves forward with it. Suddenly a three-and-a-half inch folder behaves like a two-and-a-half inch detail knife, with your finger doing the steering instead of your wrist.
Carvers, whittlers and anyone doing food prep out of a pack use this constantly. It is also the safest way to apply real pressure to a short cut, because your hand cannot slide forward onto the edge.
Every millimeter of choil is a millimeter that is not cutting edge. On short blades that adds up fast, and the notch itself can hook cardboard or rope mid-pull.
If most of your cutting is long draws through packaging, a plain plain edge with no choil will serve you better. If you do fine work, the choil earns its space.