http://en.wikipedia.org/wiki/Power_Management_Unit

For portable computers, the PMU is responsible for coordinating many functions, including:

• Monitoring power connections and battery charges
• Charging batteries when necessary
• Controlling power to other integrated circuits
• Shutting down unnecessary system components when they are left idle
• Controlling sleep and power functions (on and off)
• Managing the interface for built in keypad and trackpads on portable computers
• Regulating the real-time clock (RTC)

The PMU controls almost every power-consuming function in an Apple computer. It is constantly running diagnostics on the various power-related operations and checking them against the current Energy-Saver settings, allowing the PMU to actively manage power consumption for optimum user performance.

The PMUs functions may become corrupt over time. If this happens, it may become unresponsive and stop performing tasks. The user may not notice the PMUs malfunctions so much as the side effects of the corruption, including but not limited to:

• Failure to turn on
• Failure to restore from sleep mode
• Failure to recognize connected devices (FireWire, USB, etc.)

Resetting the PMU in these circumstances can be a relatively quick and easy fix to some of these issues. There is a keyboard shortcut on newer Apple laptops with an internal battery, nicknamed "SCOP". This stands for Shift Control Option Power. This "reboots" the PMU software in order to get it working as it should. For Apple laptops with a removable battery, resetting the PMU involves unplugging the power adapter, disconnecting the battery, then holding down the power button for five seconds. Another PMU-related fix would be to reset the logic board, where one removes the backup battery on the board for a few minutes, then reinstalls it, causing the PMU to reset itself with clean, fresh parameters (that need to be corrected, if desired, to its previous state) during the next Mac OS boot (for typical PC users, this is similar to "resetting the CMOS").

The PMU is very sensitive and a reset may be necessary if a backup battery dies. Even plugging in one's laptop in the wrong order can cause issues (power into the outlet first, THEN power to the computer). Never turn off an attached UPS without first unplugging the AC adapter.