From Apple, it seems to answer the question:

About the Power Manager

The Power Manager is an integrated circuit (computer chip) that is usually on the logic board of the PowerBook and iBook. As the name implies, it is responsible for power management of the computer. It controls backlighting, hard disk spin down, sleep and wake, some charging aspects, trackpad control, and some input/output as it relates to the computer sleeping.

Over time, the settings in the Power Manager may become unusable, which can result in operational anomalies with the computer. Examples include not turning on, not waking from sleep, not charging the battery, or not seeing the AC Adapter, among others.

Resetting the PMU is not intended for resolution of a stall or situation in which the computer is unresponsive. A PMU reset should not be necessary except as a last resort in cases where a hardware failure of the power management system is suspected. Performing a PMU reset returns the iBook and PowerBook hardware, including NVRAM, to default settings and forces the computer to shut down.

For most situations, a restart is sufficient. If the computer has stopped responding, try these steps, in order, until the computer responds:

Force Quit (Option-Command-Escape)
Restart (Control-Command-Power)
Force Shut Down (press the power button for 10 seconds)

Only perform a PMU reset on the computer if none of the above steps resolved the situation. If this is the case, see below for instructions on locating the reset button or key combination for your model of iBook or PowerBook. If your computer is earlier than the models listed here, see "PowerBook: Resetting Power Management Unit (PMU)".