So I did a search for “power assertions os x” and frankly came up pretty blank. It seems Apple’s documentation mostly consists of reproducing chunks of the header files. Not to worry, they’re fairly easy to understand, so here for the benefit of future generations is a simple example…
Power assertions are a technology for preventing the Mac from going to sleep automatically. Mountain Lion is more aggressive about doing this, so applications need to tell it which operations shouldn’t be interrupted. He’s also posted a simple Cocoa wrapper for the relevant I/O Kit APIs.
Update (2012-08-09): Keith Harrison:
The pmset command (
/usr/bin/pmset) provides command-line access to many of the power management settings that are visible in the Energy Saver preferences pane. In addition it provides a way to view and manage power assertions. For example if I use iTunes to play some music it allows the display to sleep but a power assertion prevents the system from entering the idle sleep mode.
It can also show you which processes are preventing sleep.