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Poise is surprisingly similar to balancing wheel rims. You know when you add small lead weights to car wheels to make the weight even all round?
Well, the same applies to watch balance wheels. The effect of a “light spot” or “heavy spot” is that it will affect the rate when in a given position (dial up, crown down, etc.) and therefore will make the timekeeping inconsistent.
On a modern balance wheel which has no screws, you can only remove weight by nibbling into the rim.
Sounds reasonable. But I doubt the Helson will need poising once serviced. It’s quite an involved process to poise a balance, and often not performed brilliantly. I’d prefer to leave a modern watch as it left the factory as far as poise is concerned.