Depends how you control the scapulars to start with.
In any case, you need something behind all that: Some pro boxers I’ve watched do “reverse benchpress” whereby they raise their chest to the bar which is left on the rack. Others I’ve seen lock the shoulder blades almost to touching point and then do push-ups (often wall pushups) unti they can’t stop the shoulders blades from fanning out. It’s quite a precise movement.