That is true for external pumps. Power consumption will drop when throttled back.
I have found it does not work near as well for submersible pumps though.
I went through some testing last year when deciding submersible vs external for my pond. Throttling back submersible pumps did very little but lower the flow. This is what I found last year: A mag 5 dropped about 5w when I stopped about 50% the water output, (still used about 93 w) and dropped 12w with the output completely blocked. With a 290w sump pump it also dropped 5w drop at half flow and 10w fully blocked. A little 15w powerhead was still 15w at 50% and 14w blocked.
You can get a basic plug in watt meter for under $30 and they are pretty useful for testing items all over your house to find out the actual draw (like 12,000w of Christmas lights)
http://www.discountpv.com/meters/p4400.htm
