This.
Added, the company never has to open up F/O seats in a bid if they are hiring... all they have to do is have Captains bids. They just throw new hires where they need F/Os, and current F/Os (especially on the 757 who want to move) will only get to move if enough F/Os in the seat they want move as well.
That way, there is never a bid that will trigger SDP even though a new hire way junior to you gets trained before you. That was how a lot of us got POP while monkeying switches in the back of the 727... company ducks out of a lot of passover pay, and pilots miss out. Theoretically, a guy could get stuck in the 757 for a really long time, if the company doesn’t open up F/O vacancies.