Double Minus Operator

Also known as the double unary
This can actually be used with any formulas and functions.
The double minus (--) can be used to transform TRUE and FALSE values into there corresponding 1 and 0 equivalents

-(true)
-(false)
--(true)
--(false)

It can infact be used be used with any formulas.

only be used with array formulas and is commonly used in conjunction with the SUMPRODUCT function.
This can be witnessed from the Evaluate dialog box.