The children's carriage drives over the steel cables.
A string, lengthened with elastic cord runs from the carriage to the wall. The string is amplified, and because the amp/speaker hangs under the carriage as a counterweight, the construction produces feedback sounds. The voltage produced by the speaker is amplified bya voltage amplifier that's mounted on the carriage, and supplies a permanent magnet motor.
This pulls the fifth wheel, that in turn pulls a rope that is stretched between the 2 walls. So the carriage drives over the cables; the tension on the string changes, a new feedback sound has to be generated; the carriage starts driving; etc. When it has reached a wall, another rope pulls the poleswitch of the motor, and the carriage drives backwards, and so on.