Correct me if I'm wrong. It's the spacing that is throwing most people off.

@a =+ @b-2 is the same as @a = (+1) * @b-2

if i were to write this in my code I would format it as

@a = +@b

but i wouldn't do that since +1 times any number yields that same number

Note that they mention a pre-increment operator but they mean a Compound Operator. Also note Compound Operators are only availabe in SQL 2008. Writing @a += @b-2 would throw a syntex error in earlier version.