A few nights back I was sawing the side aprons for my Stickley Lost Side Table to length. My procedure is to mark all of the faces using a knife, nip out a small starter notch for the backsaw, and then saw away the waste; it’s pretty standard stuff.
Knifing in the lines does a few things: it gives you a clean entry line on the shoulder, it makes it easier to track the saw, and if you cut things fat, it gives you a nice place to register a chisel when paring. One thing I discovered is that it also lets the waste piece break away before the saw gets to the bottom of the work piece.
In the picture above, notice the little shelf of wood left on this piece of waste? The top of the shelf is where my knife scored the wood fibers. The width is the width of the kerf on my backsaw. I looked through my other waste pieces, and this only appears when your cut is square and plumb, that is, when you’re exiting the cut with the saw right against the marking lines.
If you see the shelf, you made a good cut.