Why would that renew your miles if no miles post for that?
I didn't explain myself correctly.
With some mileage programs, when you have say 51k miles, with 45k that are about to expire, you book an award ticket with 50k, and once the ticket is issued (and the miles deducted from your account), you cancel the ticket, and a new 50k miles return to your account, which have the longest expiry date available to the program.