There is no HDD on the market that changes its rotational speed whilst continuing to spin, its only ever in 4 states. 1 = Off (or idle but still not spinning), 2 = spinning up, 3 = spinning, 4 = spinning down.
While I completely agree that WD's marketing spin (duh) is a fine example of evil marketing, purely dishonest and bad for the customers, there's actually a drive that can slow down from its standard 5700/5940 RPM to app. 4500 or 4900 (low RPM idle), depending on the model. The first and third links are lengthy docs, so simply search "RPM" for the meat.
5K1000click 1click 2
Mind you, HGST claims only selected models feature the "low RPM idle" mode. Apparently, the Deskstar 5K3000
reviewed by SPCR didn't. So, apart from the information confirming this feature, one would yet have to actually see this spinning down in practice.
Funny thing, HGST seems to disregard this information (actually spinning down, not just CoolSpin) in their marketing altogether. Very few reviews exists and, curiously enough, they also don't mention this feature.
I can only hope SPCR and/or storagereview do a test of such a drive.