Chest up!

From the left: back not in extension, cued to "lift your chest," back in extension (note wrinkles in shirt).

“‘Lifting the chest’ is an important thing to learn how to do. It is something you will need to do often in the weight room, with every exercise you do. It is the way you get your upper back into ‘normal anatomical position,’ the position in which your spine is safe while bearing a load. It may be confusing for some, who may interpret this as making the torso angle more vertical. Imagine someone touching you on the sternum and telling you to “Lift this up.” And then realize that this “chest” movement can be done no matter where the back is relative to the floor.”

--Mark Rippetoe, Starting Strength, 2nd edition