We can basically have a window which waits a set time before going to sleep.
So, when traffic is going, we can hold off on going to sleep and achieve the high throughput. Once iperf starts for example, we won't go to sleep, but when iperf stops, we will sleep as expected.
Is this what you are asking? Let us know if you need more details.
What you say makes sense. I think it's fair to assume then that this powersave mode does not explicitly care about the DTIM period of the network.