These cups have an outer surface that is a helical gear. Each horizontal cross section (like the top surface) is normal gear. Each layer beneath is rotated slightly such that makes the teeth form a helix. Think DNA double helix, except with 12 and 20 teeth.
The second helical cup is also a cage gear. This means that the teeth are freestanding and only connected at the top and bottom. It is surprisingly strong and has very little deflection when squeezed firmly. The removed surface would not have contacted the opposing gear and therefore has no effect on smoothness when removed.
The cage gear was designed to hold your toothbrush so that it can drain extra water. The first cup (which I call "closed") is designed for pens at your desk.
Uses 2 standard skateboard/fidget spinner bearings. 608ZZ Shielded 8x22x7 Bearing
2 cups not enough? Check out this modular version that can handle as many cups as you want! Thanks tbutters! https://www.thingiverse.com/thing:2854179