Probably because sour would make your lips pucker? I think lemons would be more obvious.

What happens when you wear one?

How would someone get started doing this?

That’s what caught my eye as well! Anyone who’s worked with qc or metrology would probably not be able to miss it.

It’s a flexible coupling for a drive shaft. It probably does show up on stepper motors for 3d printers. It’s used to allow for some misalignment between the motor and the rest of the drive assembly so there isn’t any damage to the motor bearings or binding of the shaft.