=sumproduct() likes to work with numbers. The -- stuff changes trues and falses
to 1's and 0's.

Bob Phillips explains =sumproduct() in much more detail he
http://www.xldynamic.com/source/xld.SUMPRODUCT.html

And J.E. McGimpsey has some notes at:
http://mcgimpsey.com/excel/formulae/doubleneg.html

SteveM wrote:

What does a double hyphen do in a formual?
e.g. =SUMPRODUCT(--(Sheet2!$A$2:$A$100=$A2),--(Sheet2!$H$2:$H$100="H98"))

I saw that as a response in a different post, and I've seen the "--" in
other responses but have not been successful in finding it in help

--

Dave Peterson