They are a method of coercion, which is a way to convert a Boolean value (TRUE or FALSE) to its numeric equivalent (1 or 0). There are several ways to accomplish it, (adding a zero, or multiplying by 1 as examples). One of the, if not the, fastest and most efficient ways is to use two negation operators together, which changes the sign twice, resulting in a numeric value.


I suggest you Google around for SUMPRODUCT and "unary operator" you'll find lots of answers(personally I use the * operator instead of --,)