I like to use my circular saw to rip down sheets of plywood. But if the cut-off piece is unsupported, it can sag — sometimes even breaking off and tearing the veneer before I finish the cut.
So to prevent this problem, I came up with the support shown in the photo. As you can see, it’s pretty simple — just a thin strip of 1/8″ hardboard with a couple 1/4″ dowels spaced 1″ apart.
After starting the cut, I slip the support into the saw kerf, and it keeps the cut-off piece of plywood from sagging ( Fig. 1).