An optical sensor passes through this mini hourglass, digitising light transmission and eventually triggering the rotating mechanism to flip the thing 180 degrees. It also can send the optical sensor's values to a PC by USB providing random numbers.
There are some good non-deterministic random sources disclosed on the Internet. They make use of radioactive decay, radios tuned to secret frequencies where there is no broadcast, dice rolling down a chute, and even lava lamps. The USB Hourglass compares well with these sources in terms of bit-rate, cost, safety, reliability, and simplicity.