This is a segmented bitholder. The segments screw into each other and prevent the inserted bits to fall out. The thread is designed pretty tide to prevent the segments from losening. Sometimes if the printer isen't accurate enough it can appear a little to tight for the first assembly. Just screw it together a few times and it will get better or recut the inside threads with a M12 thread cutting tool. I just updated it now there is a 18 bit per segment version in addition to the 12 bit per segment one.