[icegreentea]

You're right, it's not standing waves, being used a delay line. You make it 'persist' by sending a signal down the delay line, reading it out the other end, and regenerating the signal into the delay line. This is different from a standing wave, since a standing wave results from the reflection of a wave from the ends of its transmission mediums, while in delay line memory, reflections would probably be a really bad thing.

Relatedly, if you ever had an old school analog sound mixer with sound effects, it would have a spring delay line for delay and echo effects. If you were clumsy like me and ever dropped one, you could here it boinging around.