To get the feel of the forward and back movement in your chest, cross your arms at the elbows in front of you, then try to touch your elbows behind your back (which you won’t be able to actually achieve unless you are a contortionist!). Repeat moving your arms a few times in order to feel the muscles working in your upper to mid-back and the stretch across your chest. Next, anchor your hands on your hips and attempt to get the same range of motion using only your back muscles.
To get a feeling of the up and down movement, anchor your hands at your hips and take a big breath in, filling up your whole chest. Feel how this pushes your chest up. Now, take another breath in, but this time make sure that your shoulders remain back and down, relaxed, and that your head does not move. In one quick push, exhale and see how your chest drops, but do not allow your shoulders to collapse in.
To put this move together, push the chest forward using the back muscles and begin breathing in, continue breathing in to fill the chest and lift it up. You may slightly tuck the chin to the chest, keeping the shoulders back and down and the upper and mid-back muscles pushing forward and up. Slowly exhale, releasing the upper back muscles so the chest moves back. Continue to exhale until all the air is out of your lungs and your chest has slightly dropped. Try not to ever let the shoulders collapse. Practice this several times with the breath to get the feeling and then attempt without the aid of the breath. Finally, let your whole upper body flow with the movement.