Well, here's how it works.
When Disney feels that its overall occupancy rate is down, it will offer a sale. But this sale isn't a total sale all across the board, it's selective.
Whether or note a discount code is offered depends on the target occupancy rate that Disney sets; it sets a different target rate for each type/level of room at each resort. Before the sale starts, if the target rate has already been achieved, then that code can't be used for that type of room at that resort. If the target rate has not yet been achieved, then the code can be used.
If you have an existing full-price reservation and a code comes out, you can check to see if you can use that code. There is absolutely no guarantee that you can use that code. And having that pre-existing reservation gives you absolutely no advantage in being able to use the code or not. In other words, making a reservation specifically in hopes that a code will be announced (and if you would cancel the reservation if no code is announced) makes no sense, since having that reservation doesn't put you in any sort of line ahead of anyone else without an existing reservation.