Alternatively , the underclock option -1 , -2 do the opposite. It's useful for games which don't make use of the full potential of the EE. so we're decreasing it's clock speed which in turn means we need less resources from the processor to actually emulate the EE.
to dumb up my explanations, here are examples of the scenarios ideal for using the underclock / overclock options:
Underclock:
usage of EE on the game : 70% (it's clearly not used fully)
default cycle rate : 100% ( we're emulating the EE at it's default clock speed so the remaining 30% is just waste of processor resources)
set slider to -1 : 70% ( we're emulating the EE with a naturally lower clock speed which should mean lower requirement of processor resources and we're happy since it's enough for the following game and we're saving some resources which should yield a good speedup. whee !

)
Overclock:
usage of EE on the game : 100% ( clearly the EE seems to be pushed to it's limits and which in turn makes the console to underperform at certain scenarios due to this bottleneck)
Default cycle rate: 100% ( In this case underclock wouldn't help since the game needs more from the EE so Default cyclerate would reproduce the same behavior as the console)
set slider to +1 : 130% ( we're increasing the emulated clock speed which would mean need for more processor resources but the EE can actually perform better at these scenarios due to the increase in clock speed ! so when you have a processor which can handle these higher demand in resources you should see a benefit. since you're emulating a whole virtual state of the console with a higher clocked processor which could cope better on demanding scenes. )
If you still have any further doubts I'll give you some RL practical example referencing to something similar to this.