When you need to do something if you have the memory to do it and a fast processor to do it quickly then the work to do is reduced and the time it takes is reduced meaning the load on the battery is short and sharp instead of long and drawn out.

When the phone is doing nothing the processor runs in low poer mode.

Obviously if you thrash the processor so its always working then a lower speed processor uses less power and lasts longer but the idea is to not thrash it.

The idea behind it is that from one battery charge a faster processor can do far more compute cycles than a slow one. If they have comparative idle loads on the battery then you can get more battery life out of a faster processor as any actions are quickly dealt with.

Thats the theory anyway, relies on the difference in power consumption being less than the difference in cpu cycles.