Overclocking enthusiasts speed up a cpu by running it at a faster frequency. It causes the chip to get hotter than normal. This problem arises due to transistors not being able to snap instantly between On and Off. Instead the faster frequency causes them to spend a larger percentage of cycles traveling in a middle range, wasting power and generating heat.
If the devices were only totally Off or totally On, they'd tolerate being driven at the higher frequencies, with no additional heat production.