Question about entering power save mode

Hello,
 I have a question about entering power save mode in Linux. I am
talking specifically about
wireless nic drivers, but the answer is probably
the same for other devices.
My wireless nic driver does support power save mode.

What triggers the device to enter a power save mode ? is it some user
space daemon?
And in case it is - which user space daemon is it ?

Or is it some kernel thread / mechanism?

Is it the same also for desktops ?

Regards,
Mark