as an engineer, i can look at a sword blade as a cantilever beam, supported on one end, and loaded at the impact point, ie. the 'sweet spot' nearer to the unsupported end.
this places the spine of the blade in compression, and the edge in tension.
somewhere between the spine and the edge is a neutral point that is neither in compression or tension, and and cut-outs put along that line, the 'neutral axis' will not affect the mechanical strength of the beam.
it is thus possible to cut a channel in, or even pierce a blade with very little effect on it's strength.
rounded edges in lieu of sharp corners, of course, to avoid stress raisers, and kept to a reasonable size...
calculating this line would be fun tho, the ancient smith would have to have a lot of experience and experimentation with resulting failures to figure it out.
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