I think that the 'outside' approach is done because this is reverse-kerf lining, so doing it on the outside puts the kerfing's solid band of wood against the side (like regular kerfing is installed) which makes the bending easier with less cracking of the kerfing. Just dampen the kerfing and bend it. However - I now bend the reverse kerfing over a hot pipe to fit it inside. I clamp it down with clothspins to let it dry from the wetting/hot pipe bending. Better bends, less cracking, and I can better fit the kerfing over the inside reinforcement veneer for the side sound port which I run top to bottom, underneath the kerfing.