setting up an electronic load
For most testing constant current is the easiest mode to use. You can set
the current to the appropriate current level and verify the output voltage
specification. 


Constant Current -- the load will try to sink the programmed current
Constant Resistance -- the load will try to sink a current to maintain
a programmed resistance. This is done my multipliers in the control circuit.
Constant Voltage -- the load will will increase current to maintain a programmed
voltage. This is useful when trying to find the current at which
the power supply folds back (knee current). You set the load to the foldback 
voltage and then measure the current. It is quicker than stepping the current
but does require a more complex control circuit.


Since most loads are current sinks you shouldn't be able to damage it when the supply
is putting out low voltages. You could damage the load with too high a voltage
but for an ATX power supply this is unlikely. The output stage of most loads consists of
power MOSFETs and these typically have breakdown voltages of 20V and above.

Some power supplies have minimum load requirements for the individual outputs.
A comprehensive test would load all the outputs at various combinations of min and max
loads but you could probably attach the load to one output at a time. Make
sure that each of the other outputs has the min load.