Question

To help you steady a rifle when you're ready to shoot draw a deep breath and

Answer 1

Let half of your breath out, stop breathing, then shoot

Answer 2

To help you steady a rifle when you're ready to shoot draw a deep breath and exhale about half of it. This is one of the Rifle-Firing Techniques. This technique helps you steady the rifle for the most accurate shooting. 
After you exhale about half of the breath, you should hold your breath as you squeeze the trigger.