Graham
I've just bought a Makita Ls1013 sliding mitre saw. Supplied with it
is something called a "Kerf Block" which is semi circular and
supposedly screws just behind the fence in the middle, in line with the
blade.

I would suspect its a splitter or riving knife equivalent. I'd guess it's
to stop the cut closing up when its with the grain. A 'kerf' is the width
cut by the blade.

I'd imagine its meant to move with the blade.

Or maybe I'm talking complete Jackson Pollocks not having seen
it...............

Dave