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There are several kinds of BCDs, or buoyancy control devices. Neck ring, Wings, and the most common in sport divers, the Jacket stile. Nevertheless they are designed to provide ample buoyancy at the surface and to help the diver to control and maintain neutral buoyancy throughout the dive.

To do so, the BCDs need to be inflated and deflated depending on the depth and other factors. Since the BCD is an air bubble and it responds to physical laws, the air will always be at the highest  possible point, trying to escape towards the surface. Therefore, in order to deflate a BCD the diver must use the valve that is closer to the surface depending on its position.

The diver on the picture is not doing so because some air will stay trapped on the shoulders of the BCD.  This is a very common mistake very easy to correct. Visualization techniques and using the different valves installed in the Jacket, will spare some divers a quick ascent to the surface.