Drilling it out, then backfilling with the putty type cut paste to avoid rot, allows the swelling to swell inwards rather than outwards.
One of the issues that occurs with the “chop” method is Zelkova swells as it makes callous. Good brooms have a nice continuous “sweep” up as they transition from single trunk to multiple branches. No bulge.
But when we chop, the healing process creates a bulge. So, one way is to use raffia or pipe clamps to prevent the bulge. The thing is, the tree produces the callous regardless! So, hollowing out the trunk allows that callous to grow inwards not out.
Next, after you get a bunch of branches growing, there’s going to be a gap between them. That gap will eventually close up. But now it will look like every branch started at the exact same height. So, what Ebihara did (there I go parroting again) was to have the core hollowed out so that he could carve down between the new branches. Make a V cut between them. And the depths of the Vs could slightly vary, giving the tree a more natural look.
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The pictures go from right to left chronologically.