Broken search: sorry, I left a bug there. Fixed now. About to commit next version (it has features too!).
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Re-compute normals: I know, it cannot be perfect all the times, it is just supposed to be good enough, most times.
So it is just a tool intended to save a lot of time in certain common situations (you can also do in in bulks).
But surely it cannot magically substitute careful, manual edge-by-edge selection of "smooth/hard" status.
That is supposedly done with whatever application you use to edit your meshes.
If the mesh you import has normal information, that's gonna be preserved.
What it does: first, it trashes the normal you currently have, and compute them for each face anew. Then, if makes edges "hard" or "soft" according to the angle of the two faces around that edge: almost flat solid angles are made soft, steep angles are made hard. In reality it is more complex than than, (there's vertex replication and vertex joining involved, it takes in account texture seams... and so on), but that's basically the idea.