The Cork Screw**

Breathe in and lengthen up through the spine. Zip and hollow
(Breathing out): float your arms up like the wings of an eagle until they come to rest behind your head.
(Breathing in): Shrug your shoulders up to your ears.
(Breathing out): Drop your shoulders back down.
(Breathing in): Gently bring your shoulder blades together. Your elbows will go back, but not out of sight.
(Breathing out): Float your arms back down (like an eagles wings in reverse). Let your head and neck lengthen up as your arms go down (like the cork out of a bottle!).
Repeat three to five times.

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It's called the corkscrew because the last part feels like a cork being cork-screwed out of a bottle!
This is a great exercise to do if you have been desk bound for some time, or you have been carrying a lot of shoulder tension. It is a little complicated, but worth getting right.

What it does

Works the muscles that run between the torso and the shoulder blade.
Encourages the Scapula to move in correct ratio with the humerus (the "scapulo-humeral rhythm"[1]).
(Assuming you are applying your scapular anchor): Specifically works two muscles that are very commonly weak and elongated beyond their ideal functional length. These muscles are the Lower Trapezius and the Serratus anterior(1).

Watch Points

Do not arch your back as you bring your scapulas together.
(Except for the moment of the shrug), do your best to maintain the scapular anchor.

Reference

Shirley A Sahrmann: Diagnosis and Treatment of Movement Impairment Syndromes; Publ. Mosby 2002
The Official Body Control Pilates Manual Available from: http://www.bodycontrol.co.uk/