Chris, the riving knife or a splitter (they are two different tools that do a similar job) prevents wood that has stresses in it froom closing up behind the blade. Without a RK or splitter, the wood could close up, pinch the blade at the back, the blade picks up the wood and throws it back right at where you are standing, whacking you in the face (or other parts of your body).
That about sums it up I think.
My diet is working good. I'm down to needing just one chair now.
"Just think how stupid the average person is, and then realize that half of them are even stupider!" --George Carlin