The upper back extension isolates movement in the thoracic spine, which is often tight and weak and more difficult to train well if the lumbar spine is also being flexed and extended.
 
From on your knees, support your abdomen on a bench or similar to position your trunk horizontally, and place your fingers at the sides or back of your head like you would for a sit-up. Keeping the lower back and abs tight to minimize movement, flex the upper back to curl down, then extend maximally to bring the shoulders and head up as high as possible. Hold momentarily before repeating.
 
Weight can be held in the form of a dumbbell behind the neck.
 
The exercise can also be done lying on the floor to focus only on extension from neutral.
 
Purpose
The upper back extension strengths and mobilizes the thoracic spine.  

Programming
The upper back extension should be performed for sets of 10-20. It can be done multiple times in a week if the upper back is a problem area.
 
Variations
The upper back extension be performed supported on different piece of equipment from a bench, exercise ball or even a barbell in a rack. Slower tempos and longer pauses can also be used.