Retraction allows you to work the most muscle fibers in the chest. If you cannot retract your shoulder blades due to bad posture, you must strengthen the muscles that do so.
This means you need to perform rowing exercises with proper form and stretch your chest and anterior deltoids if you have forward shoulder posture.
Lower the weight all the way to your chest and pause.
If you can't lower the weight all the way to your chest, you do not have to. Just make sure you are not being lazy.
When you lower the weight all the way to your chest, your pecs are actually not working as much, but it's okay, because you will more than likely be doing more chest exercises which isolate the pecs.
The sticky point is the point in which you know the lift will be completed. Hold your breath until then.
If you exhale before you pass the sticky point, you will lose all intra-abdominal pressure, lose the optimal bench press form and not get the full benefits of the bench press.
Personal Trainer Michael Behnken, MS, NASM-CPT-PES, CSCS