What is a CUDA switch?
CUDA is officially called the CUDA Microcontroller Chip, which handles powering on and off functions, system resets, parameter RAM, ADB, USB, FireWire and serial ports, and the clock.
As for what CUDA actually stands for? I honestly have no idea :)
This term is interchangeable with PMU switch and resetting it is also known as "resetting the logic board/motherboard".
This switch resets the Power Management Unit on the motherboard. According to Apple, the PMU does several things, including telling the computer to sleep, wake, idle, maintaining the system clock, maintaining various PRAM values, etc. (Source: http://docs.info.apple.com/article.html?artnum=95037).
Sometimes if your Mac is having problems booting, sleeping or waking, it can be useful to reset the PMU using the switch (the procedure is detailed in the linked article above). Apple advises not to press the switch more than once at a time. Pressing the PMU switch will reset PRAM.
A picture of what the switch looks like and where to locate it can be found here: http://www.cubeowner.com/gallery/parts/Mainboard01.
The advice to press the CUDA switch only once is to ensure folks don't interrupt the power cycling that goes on while the cuda microcontroller chip is being reset.
Pressing the switch interrupts the constant electrical flow to the controller chip, and if you press it more than once while it's cycling, you can presumably mess up it's reprogramming cycle.
The other thing to keep in mind is that the Cube MUST be powered down (shut down) prior to pressing the CUDA switch. Power cord should be still connected, and battery in place.
