What exactly is "breaking in" on a new cartridge ?

me too theo - my cartridge makes the biggest improvement when it is warmed up.

What they said, plus your ears and brain.

I am breaking in a Ortofon 2M Black

Those (burn-in, break -in) are frightening expressions or phrases considering the fragile nature of the objects involved. In Europe we use the expressions 'play in' or 'warm-up'(time) which are obviously more adeqate for the purpose.

No.

New piston rings, new semi-auto handgun, etc., these
things need mechanical "wearing-in", so they loosen up and
operate with less friction.

What is happening to a brand new cartridge during it's
"break-in" period ? I do not mean "warming up" for
regular daily playing.