This is my data:
1 10
2 20
3 30
4 40
5 50
The formula:
=SUMPRODUCT((A1:A5<3)*B1:B5)
returns a value of 30.
I assume this is because it searches for all those values in column A where the value is < 3 (ie the 1 and the 2) and sums their correspondng values, ie 10 and 20...