The belt grips tightly to the focus or zoom ring, and there is no chance of slip. The tension can be adjusted to your liking. The belt has grooves in it that match the pattern of grooves on the focus/zoom ring. This provides a non-slip connection.
The belt is made from industrial strength fiberglass reinforced neoprene, with a pattern of grooves on the inside. The only way it will break is if you intentionally cut it with a knife. It is very strong.
The Shifter and Focus Marker Board will both be made from high quality, durable, injection molded plastic. Not the cheap stuff that toys are made of. There are also some metal parts inside that are CNC machined. The video shows it being made on a 3D printer, but that was just showing the process that was used for the prototypes.