Posture
Stand with arms stretched above your head 
Stand up straight and stretch your arms above your head.
Lower your shoulders – maintain the tension in the back 
Keep your arms stretched and lower only your shoulders until they lie flat behind your back.
Slowly lower your arms to the sides
While maintaining this position, slowly lower your arms to the sides.
Upright posture with noticeable tension in the back 
Practice singing with this upright posture.
Breath out while maintaining the upright posture
Now, quickly breathe out all of the air in your lungs and sense how your back muscles are activated to maintain this upright posture. You will also notice that your stomach moves inwards. Between these two movements, you will notice a kind of "pulling" effect at your breastbone.
Let the air drop into the stomach
Now, simply let the air drop into the stomach without changing your upright posture and notice that, together with the "stomach feeling" to the front and outwards, you also feel a strong tension at your back.
Correct posture
If you are doing everything correctly, the pelvis will move forward accordingly…
Incorrect posture
…thus preventing a "hollow back" posture.