Cueing with - open your chest doesn't always cut it when it comes to mindful movement and relief for muscles like pectoralis minor.

No worries if that's how you cue. It's a starting point. Much like using the ever so common door stretch for addressing chest tightness, it is helpful. 

The pectoralis minor is a shoulder muscle that connects the scapula to the ribs, effecting scapular motion and breathing when the scapula is fixed. It is active anytime we need stabilization of the shoulder girdle with the arms in front of us.