To look your best on stage, you need to be able to position yourself in the best angles possible for the judges.
For the front and rear bicep poses, you need to be able to position your shoulders, elbows and wrists at 90 degree angles. Personally, I have very tight shoulders (which restrict me on both the poses and my bench and squat positioning). So, in posing, I compensate by flaring my rib cage and therefore obscuring my lower back definition.
These wall slide exercises help to gradually extend your scapula through a full range of motion. When performing them, the rib cage must be held down (not flared) and forearms remain against the wall.
I perform these as part of my warmups on every upper-body training day; give them a shot yourself and let me know what you think!