It's quite simple, really. Most of a drive's power is consumed by the motor that spins the disk inside the drive. Reduce the speed of the disk, and you reduce the amount of power required. However, Western Digital doesn't want to say that they're selling 5,400 RPM drives — those became second class in the desktop market years ago. Instead, they rate the drive's speed as "IntelliPower" and take pains to emphasize that there are other factors that affect performance.
Western Digital has caught a lot of flak for withholding the rotation speed of the Green Power, especially when the product was first launched and the marketing material listed the rotation speed as 5,400-7,200 RPM. This led some to speculate that the rotation speed changed dynamically during use — which would have been an impressive engineering feat had it been true. The reality is revealed by a sentence that Western Digital added to the description of IntelliPower: "For each GreenPower™ drive model, WD may use a different, invariable RPM." In other words, Western Digital reserves the right to release both 5,400 RPM and 7,200 RPM drives under the Green Power name — without telling you which are which.
We were able to confirm that our 750 GB Green Power had a spindle speed of 5,400 RPM by doing frequency analysis on a sound recording of it. Why sound? Sound is vibration; the pitch of the sound corresponds to the frequency of the vibration. Hard drives vibrate at the speed of their motor, so they produce a noise at the same frequency as their rotation speed. Our sample had a sharp spike at exactly 90 Hz (cycles per second). Multiplying that number by 60 (to get cycles per minute) yielded a measured rotation speed of 5,400 RPM.
It's possible that other Green Power models use a higher spindle speed — but we doubt it. Storage Review tested the 1 TB version of the drive and determined that that model also spun at 5,400 RPM based on a calculation of the drive's latency compared to a previous Western Digital model. That leaves the 500 GB model — which Western Digital says is even lower power than the larger capacity versions. With the majority of the Green Power's efficiency advantage coming from its lower speed, it seems impossible for the 500 GB model to use a higher rotation speed. It's possible Western Digital intends to release a 7,200 RPM version at some point in the future.
(finally, yes, they took time n a lot of bad press in the meanwhile 
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