Old thread I’m sure it’s been said, but any type of stretch movement. I always had a big side chest, always did heavy pressing until I injured my right pec a bit and it nags, I started adding a TON of stretch work. So DB flys, cable flys and even presses you can get negative on. Some machines I’m able to do it, not only has my flexibilty in my chest gotten 10x better. My presses have gone up and my chest has blown up.