This type of breathing is generally done in combination with thoracic breathing in periods of great stress such as strong physical exertion or obstructive airways problems such as asthma or emphysema. The upper ribs and collar bones are pulled upward by the sternum and neck and this allows more air into the lungs. In yoga we only use it alone to create awareness and then afterwards combine it with thoracic and abdominal breathing to form yogic breathing.
To practice clavicular breathing one starts with breath awareness and then thoracic breathing for a few minutes. After that breathe in fully, into the chest, from there try to breathe in a little more so that one can feel the expansion right into the upper lungs. The collar bones and shoulders will raise up slightly. Next exhale, first relaxing the neck and upper chest and then the ribcage. Continue a few rounds.