So simple! A flat piece of wood with a fence on each side, the bench hook grips against the bench when any pressure is applied away from the operator (of the saw, plane, etc). One side is optimized for crosscuts, and the other is optimized to be used as a shooting board.
The strips here were crosscut to the same general length with one side of the hook, then shortened by minute increments using the hook's other side. A crosscut saw makes the hook act like a miter box, and a plane turned on its side makes the hook act like a shooting board.
Here a roughly lengthened strip is about to get fine-tuned.
Like so. That little rabbet (the recess on the right side of the fence) simply allows my thumb to grip the plane comfortably. Would it be easier and more appropriate to be using a low-angle plane for endgrain? Absolutely... but alas, I only have a reasonable tool budget; perhaps my next big purchase will be a Stanley sweetheart low-angle jack plane.
The hook makes stripping the bottom of this box short work, without any power tools, and with barely any movement - it all takes place right at the bench!