I'm working on something else, but a while back, on the Himalayan Imports forum, someone published a little computer analysis of a khukuri blade. It was interesting, because it suggested that a lot of stress accumulates on the blade at the cho, and if the cho isn't there, there's a fairly good chance of the blade splitting. Because the cho is there, the blade doesn't accumulate stress on that part of the edge a crack, and therefore, the khukuri can hit with full force. The smiths would have figured this out by seeing blades split where the cho is and experimenting with putting a hole in the blade at that point to stop the crack before it starts.
While I don't think this reason is definite, it's nice in that it doesn't particularly matter what shape the cho is. The fact that there's a break in the edge at the point of maximum stress is the critical thing.
Best,
F
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