This is a method of clamping down the rib with a block lined up as a safety guard when sawing or slicing (with a knife and straight edge) off the excess length of the ribs.
A saw line, final line to which the edge will be planed, and guide line beneath as a reference parallel were drawn. Once sawn, a hand plane lightly shaves off the excess and leaves a clean line. Wet the ends of the rib wood grain before planing to soften the wood and avoid splintering.
A neat idea is to set the plane blade at an ever-so-slight angle so one side cuts slightly less than the other. Then you can move the plane to one side or the other depending on how much you want to shave off.