undervolting runs the processor at whatever clock speed you've set just with less voltage to the chip...as i understand it though, this doesn't necessarily translate to reduced battery drain. it may for some, and not for others.
also, you can set a profile so that if your device reaches a certain temp. it scales back the overclocking. I have mine set so that if it hits 107 deg, the cpu scaling goes back to stock. hopefully this will prevent me from frying my phone cause i don't have 500 bucks laying around to replace it, and i don't know if it would be claimable on my insurance.