So it seems that the explanation of the question is wrong, as it states that the pre-increment operator is not used in SQL Server. SQL Server uses however a compound operator, meaning addition and assign, not incrementing the value. It still remains unknown to me why SQL Server ignores the + in =+ instead of giving a syntax error.
Yes, explanation is wrong!
@a=+@b means simply that you assign an +(expression) to @a.
+ is unary operator: http://msdn.microsoft.com/en-us/library/aa276846(SQL.80).aspx