Follow the link... it is explained. <cross-posted>
Of course, the channel should be V-shaped, which would eliminate the slight imperfection seen in the otherwise brilliant piece of work shown. No channels would require cutting the face plate to match the resultant internal curve: more work and more curves to get right.
I'd like to be good at this sort of thing, but I know I wouldn't be. My woodworking is rough. Having made jewellery as a hobby taught me my metal-working limits. I'll sit back and admire the photos!