If you've compared single and dual versions of these cards you may have noticed that besides the CPU chip there are a bunch of corresponding large power supply components (either present or absent). These are part of a buck converter that drops the voltage of the input power (5, 12, or 28v) to the low voltage used by the CPU chip: Vcore (~1.3v). Each CPU chip has its own power supply. It makes sense that the Cube will not boot without power (VRM bypass) connected.
en.wikipedia.org
The Vcore on these is configured with a voltage divider between the Vcore and the feedback to the logic controlling the buck converter. A voltage divider is a simple ratio of resistor values, and changing the value of either resistor (within reason) can adjust the resulting Vcore.
en.wikipedia.org
One simple way to change the value of a resistor in a circuit is to add another resistor in parallel. With surface mount components, a new resistor can be easily soldered directy on top of an existing resistor. This will result in a lower resistance. If you do this to the voltage divider you can either raise or lower the Vcore, depending on which side of the divider you make the change.
en.wikipedia.org
To make it easier to change the Vcore on these boards, the later versions have the SW3 switch block added. This basically selectively connects resistors in parallel with the existing voltage divider to *raise* the Vcore. The amount of change and resulting value is all determined by the values of the various resistors installed. I've seen different values used, particularly between 7448 and 7447 versions, and between Cube and tower versions. You could look up the datasheet on the controller chip for the buck converter, measure the resistor values, and calculate the resulting Vcore. And/or you could use a volt meter to measure the operating voltage on an installed and running board.
Now as I mentioned, the SW3 block connects (or adds) resistors in parallel. So with all of these switches OFF, the card is set at the lowest possible voltage (without soldering compoonents). Turning switches ON will raise the voltage, from whatever the "base" Vcore is.
The first two switch blocks set the speed for each CPU: SW1 controls the speed setting for CPU0 and SW2 does the same for CPU1. SW3 has 6 switches and sets (or can increase) the voltage for both CPUs. Switches 1-3 are for CPU0 and 4-6 are for CPU1. Note that this is "flipped" geometrically because looking at the board, the 3 switches on the left are for the CPU on the right. Now if you look at the other side of the board, just behind that switch block, you'll (probably) see 6 resistors in a symmetrical configuration. This means that on SW3, switch number 1 does the same thing for CPU0 that switch number 6 does for CPU1. Looking at these 6 resistors again, you'll (probably) see that the outside resistors have the highest values, which means they'll have a smaller effect when added in parallel. The 3 resistors for each CPU have different values and can be added (i.e. cumulative) in different combinations to get various increases in Vcore.
Maybe you don't care about any of this and just want to "make it go," but with a little understanding of what is going on you should have an easier time than randomly flipping switches.
TL;DR:
Looking down at SW3 with position 1 on the left, all OFF is the lowest voltage and all ON is the highest. The two outside switches (1 and 6) make the smallest changes and the two inside make the largest. So the sequence of symmetrically increasing voltage for both CPUs would look like:
Code:
vvvvvv
^vvvv^
v^vv^v
^^vv^^
vv^^vv
^v^^v^
etc...
Again, this assumes your board is assembled like others I've seen.
When overclocking these boards, particularly in a Cube, temperature is important. Fortunately there is a built in temperature sensor that can be read by software (CPU Director). Rather than turn everything up to eleven and hitting go, I would suggest incremental changes while monitoring temperatures and stability.
For example, if the card is currently at dual 1.4GHz, I would try bumping one CPU to 1.5GHz. If it's not stable, then I'd set it back and try increassing the other CPU. If it were stable and cool enough, then I would bump up the second CPU to 1.5GHz, and so on... If I reach a point where one of the CPUs is not stable, but temperatures are still low, then I would try bumping up the corresponding voltage for that CPU. Changing the voltage usually means disassembly of the heatsink, so I try to "map" out the stable speeds for both CPUs before changing voltage(s). The speeds and voltages do not need to match between CPUs. The CPU chips probably came from the same "batch" but will not necessarily overclock the same.
