This is used to reduce a lot of wobble with Nophead's Z couplers if you have flats on your motor shafts. Despite the low print accuracy because these parts are so tiny, I reduced the wobble from awful to almost perfect.
The provided STL is for a motor shaft of 5mm diameter with a flat diameter of 4.5mm.
Print at the lowest layer height possible.
Insert between the motor shaft and flexible tubing.