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My theory is "friction", if that's the right word - static friction more than dynamic. Mileage may vary from one person to another due to skin differences and playing style, and also from low to high volume.

With the same aperture rounder rims give a feel of a wider aperture than do more flat and edgy rims.

Sharper edges give a better definition of attack and of pitch, but may stiffen legatos.

Klaus