How to create a offset in a bex query variable?

Hi everyone, I come across a question when i was creating a bex query. 

There are two key figures in the query, "amount of current year" and "amount of last year".Some one told me that i can use "amount of current year" to restrict "amount of last year" by creating a variable offset. Can some one tell me how to do it in detail?

Regards,

Jack