This is like a conventional fountain, with a pump that squirts water upwards and a little pond to collect the spewings. Except, it's mounted in an elevator.
A digital circuit controls the pressure of the pump, using a feedback loop, to ensure the height of the fountain remains constant.
As a result,
when accelerating upwards, the fountain appears to run in fast-forward, and when accelerating downwards it appears to be in slow motion.