It's unlikely that ABS could have wrung such impressive overclocking performance out of its Core 2 Extreme system if Intel hadn't put a lot of effort into reducing power consumption in the Core microarchitecture.
Intel's previous generation of Pentium Extreme Edition chips drew up to 135 watts of power. The Core 2 Extreme X6800 draws only 75 watts, according to Intel's thermal design specification; and the more mainstream Core 2 Duo parts bump that number down to 65 watts. As a result, ABS had the headroom to dramatically overclock its system. Moreover, the design should enable system vendors to build high-performance PCs in smaller, quieter cases.
Intel developed its Core microarchitecture from the ground up, focusing on multiple CPU cores, high performance, and low power consumption--there's a lot of technology packed onto its 65nm die (shown below). Using lessons learned in building its successful Pentium M mobile CPUs, Intel first improved its mobile line and released the Core Duo CPUs (for details on these chips see "Notebooks Rev Up With Dual-Core Tech"). Then the company set out to strengthen the performance of its desktop chips, while dramatically reducing their power consumption. For example, when your PC is sitting idle or running just a few simple apps, the Core 2 Duo can clock down or shut off parts of its logic to conserve power.
Much of Core 2 Duo's performance advantage over its Pentium predecessors comes from an additional execution unit on each CPU core. (Core 2 Duo chips have four such units per CPU core versus the Pentium D's three per core.) The additional unit per core, plus some clever coding that lets the chip fuse common groups of instructions into single instructions, allows Core 2 Duo chips to outperform Pentium D chips that run at higher clock speeds
A staggering 4MB of L2 cache keeps the higher-end Core 2 Duo chips supplied with the data they need in order to run at full speed, and Intel has carefully tuned their prefetching algorithms, which preemptively cache the appropriate data before the CPU needs it.
While most dual-core chips, including AMD's Athlon 64 line and Intel's Pentium D CPUs, dedicate a certain amount of cache to each CPU core, the Core 2 Duo provides shared access to its entire 4MB of cache. And the chip can distribute that cache between its cores as needed. If one core is churning away at a particularly complex task, it can use most of the L2 cache, while the other core runs a simple task that demands less cache memory.