Basically, each array will return a series of true & false results that could be expressed as 1s (true) and 0s (false).
Suppose the first array returns:
1 0 1 0 1 0 1 0
and the second array returns:
0 0 0 1 1 0 1 1
If you multiply the corresponding values from the two array, to find which values are true in both, you end up with:
0 0 0 0 1 0 1 0


(i.e. two true pairs). The SUMPRODUCT simply sums the 1s.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]