Comment by _flux

There are a lot of such moments, but they are just short. When you're playing music, you download a bit of data from the network or the SSD/HDD by first issuing a request and then waiting (i.e. doing nothing) to get the short piece of data back. Then you decode it and upload a short bit of the sound to your sound card and then again you wait for new space to come up, before you send more data.

One of the older ways (in x86 side) to do this was to invoke the HLT instruction https://en.wikipedia.org/wiki/HLT_(x86_instruction) : you stop the processor, and then the processor wakes up when an interrupt wakes it up. An interrupt might come from the sound card, network card, keyboard, GPU, timer (e.g. 100 times a second to schedule an another process, if some process exists that is waiting for CPU), and during the time you wait for the interrupt to happen you just do nothing, thus saving energy.

I suspect things are more complicated in the world of multiple CPUs.