For those of you who can't afford a plunge cutting saw and guide rail, here's an inexpensive home-made jig that does much the same thing.
The fellow who wrote these instructions makes it more complicated than it needs to be, but his photos show pretty well what the jig is about. He based his design on a video — which is posted at the bottom of this page.
I used this type of jig for 20+ years and only stopped when I got a plunge cut saw. It was a handy tool for breaking down sheet goods and trimming the edges of doors.
Not much that can go wrong when you use one of these jigs. There's no splinter strip (though I suppose you could add one) so the cuts aren't as clean as with a plunge saw and track. And the base of the saw isn't indexed to the jig; it's up to the operator to keep it from wandering off the fence.