OK, to understand why this works, you need to understand the camera refuses to AF with the Canon extender because it's being told there is an f8 (or f11 for the 2X) lens on the front. The software of the camera then declares "hell no".Yes, but:
The Kenko converter is passing the f stop of the lens without the change in aperture. So the camera thinks there is an f5.6 lens on it. Now you still need decent light and a contrasty subject, more than without the telecenter, but it still works.
Why the -1? Because he camera is metering at f8 while being told it has f5.6. So before the photo is taken, it is metering wrong.
The camera thinks it's metering at f/5.6 and shooting at f/5.6, while the actual metering is at f/8 and the exposure is a f/8. So - it doesn't try to control light by manipulating aperture. So why the need to dial in compensation?