Kneel on the ground holding a barbell with two small plates on the end of it with a shoulder-width grip. Keeping your back flat, allow the bar to roll out in front of you as you extend your arms and torso to follow it Go out as far as you can without arching your back, then use your abdominals to drag the bar back to the starting position.
Barbell rollout (on feet)
A much more advanced version of the barbell rollout. This time start on your feet instead of your knees and follow the same instructions as above.