I'd left the cambium layer (the pinkish stuff under the bark which covers the sapwood). With my poor colour vision, I probably didn't realise how thick it was.
I expected some thin fibrous stuff to pop off with a 'tic', and indeed, one section did.
What I didn't expect was a rifle shot 'crack' as a whole layer came off and scared the crap out of me. Initially I wasn't sure if it was the sapwood splitting, but in decent daylight I could see the colour difference and the pristine creamy yellow of the beautiful freshly exposed sapwood.
Whew, it's a labour saving way of getting a perfect sapwood back, but not for the faint hearted.
Interesting that the fracture started at a line of tiny pin knots.