Maybe it's just me, but I initially had trouble visualizing this.
Skip reading the rest of my post if you already understood how an offset woodruff key works /ubbthreads/images/graemlins/smile.gif
Basically, forget the dowel pin we use to lock the gear to the cam.
Think of a half-moon cut out of the camshaft. You then have a "key",
also shaped like a half-moon that drops into that machined slot in
the camshaft.
Like this:
So then, you can have a "regular key", a simple half-moon, that would
align with a slotted cam gear at 0 degrees.
Like this:
But, if you wanted to "move the slot", you could machine a specially
offset key like so:
That way, the half moon drops in just as before, but the exposed part that lines up with the cam gear is shifted over.